I guess first I would say that these batteries/phones are smarter than something like "greater than or less than 50% charge counts as a cycle or a half cycle". The phone's battery isn't going to lock you out of using the battery based on a counter like that. A counter like that (probably more sophisticated than that) might lock you out of using the last, what would have been, 10% of your battery, as a means of preventing too low of a voltage as the battery ages, whereby phones restart (on their own) or glitch out due to not having the voltage they require to operate. In those cases, the battery starts to operate as if the old 10% battery life mark is now the 0% mark, as an example. I think we all recall a story like that not all that long ago, where a company pushed an update that did something like this, that upset a lot of people.
In any case, I wouldn't concern yourself over waiting for your phone to die another 3% before charging it because it hadn't fallen below 50%, yet. If it was, there'd be some people who would find themselves out of a phone in a matter of months between plugging their phone in to a computer to transfer data like a USB stick, or charge for their 3mins drive to work and the 3mins back home, or charge to go to the fast food place on their lunch break, etc.
NiCd batteries were notorious for a memory effect. If you discharged NiCd down to 22% almost on every use, because that's when your R/C toys often become basically unusable or no longer fun, then these types of batteries would settle into that range and then you'd find yourself with batteries that only had some ~78% of the capacity that they used to have. And then the memory effect could start over from there! Eventually you'd find yourself with batteries that, when charged, just wouldn't last very long. It was better to almost fully discharge them. R/C guys of old would probably recall the common practice of full discharge before a recharge on these batteries.
NiMH batteries fixed a lot of the memory effect in the same form factor. NiMH was a better technology. And battery techs since have never wanted to go back to dealing with a memory effect. Full discharge isn't necessary.
Phones largely use Lithium Ion or LiPo today.
On some phones, overnight charging 24/7 - 7 days a week isn't gonna hurt the battery as compared to letting it run dead through the night once or twice a week. But that varies from company to company, as some companies may consider the battery dead when it actually has 6% life remaining that is not available to the user of the phone. The phone powering itself down at what shows as 0% to the user, but at 6% actual life remaining, could protect the battery from what otherwise could've been a lot more long term damage. These measures and how much battery buffer implemented, are largely up to the companies building the phone. Cheaper phones may not have niceties as these and companies trying to squeeze every drop out of a battery benchmark may not have niceties as these. For some battery chemistries, something like this is almost a requirement, as too low of a voltage on some batteries can be dangerous - some even igniting. Folks in the R/C hobby using LiPo will be familiar with this concept.
Usually you will need to reference the manufacture's notes on charging.
But there is something of a balance between minimizing charge cycles, and keeping the phone on a charger, that will lend itself to optimal battery life. A person who was super familiar with a specific phone's battery chemistry might even argue to keep the phone between 20% and 85% at all times for the best battery life, but that kind of stuff isn't practical for every user.
One thing that might stick around as a form of stigma, is that back in the day, the charge controllers on phones used to dump voltage/amperage into the batteries at a set amount all day, even at 100% battery charge. This would cause warm to hot batteries and it used to be recommended not to keep the phone on the charger all the time because the heat from the constant charge would kill the batteries. This is well beyond any battery chemistry's natural problems with staying at 100% charge
Now the charge controllers on these phones and the batteries themselves (with their own circuitry) are smart enough to not just dump damaging voltage/amperage (heat) into the battery all day long - they will go into a trickle charge mode which is closer to the power requirement of the phone at that time, instead of the power required to both power the phone and charge the battery. There is almost no real concern for keeping your phone on the charger through a whole night and doing that night after night.
A real concern can be long term storage, which is probably more a tablet than a phone issue. You don't want to store a device with a dead battery for a year or two. Batteries don't really like 0% or 100% long-term storage. That means you shouldn't top off a Kindle to 100% and then fully 100% power it down to be tucked away for a month, but it'll be in much better shape, and is often common-use, if it's charged to 100% and then left just in a sleep mode (where wi-fi and whatever else are still sipping power in the background)
Cell Phone batteries will die even with 100% best care taken. Generally, you should expect the battery to show noticeable signs of wear after 2 years and you can pretty well expect to need a new battery and/or phone after 3 years. And because Cell Phone companies often don't support their old hardware for significant updates beyond a few years, anyway, we really shouldn't expect to keep a phone for more than 3 years between battery life and software updates.
Personally, I leave my phones on the charge as much as possible (at home, at work) so that it's ready to be without a charge. And with that kind of use, my Galaxy S7 lasted from April 2016 up to Feb 2020 when I replaced it, mostly out of annoyance of battery life. In Feb 2020 the battery would probably drop roughly 1% per 1min of use of the most basic of tasks (it'd be more % than this per min in a game and less % per min than this on a phone call where the screen was off), and in both airplane mode and sleep mode, starting at 100% (as if it was charged in the car), I'd probably wake up to 80% charge over night in a hotel or something, where a solid battery would still probably be in the 90s of %. That's roughly a 4 year anecdotal case attempting to be as honest about it as I can.
Unless you're leaving your phone dead in a corner for a week or something with any real regularity or allowing it to die through the night regularly, I wouldn't worry too much about your phone's battery. The difference between optimal un-human/unrealistic charge of a phone and most people's every day use of a phone, has fairly minimal difference in the duration the battery will last and is pretty close to what most suggest as replacing the battery every 2 years, anyway.
If you use a wireless charger that makes your phone hot, that might be something to use as little as possible in the context of preserving battery longevity - Heat Kills!