how is batting second a disadvantage? stadium has no lights?
It is not, universally. But in some conditions it becomes so.
The matches in this world cup are a mix of day or day/night. The day matches go from about 10 am to 5pm (nearabout) local time while the day/night matches start at around 2pm and go on till 10pm. Obviously these latter need floodlights for the 2nd half of the game.
Just these different timings give you different playing conditions - chasing a target of runs under floodlights is trickier (shadows, uneven lighting, etc), so the side winning the toss might opt to bat first. But at the same time, if there's dew in the grass, the ball gets slippery and difficult to grip and spin, which might offset the problems for the batsmen.
And there are other conditions that impact - wind, rain, grass (or not) on the pitch, how hard the ground is (so balls get dispatched quicker to the boundaries), etc.
Biggest is of course mental - a team that would easily score 300+ when batting first, may falter when chasing 250; all because they let the stress get to them. And that's what has been observed in this world-cup: of the 17 matches till date, only 7 were won chasing. Of course, that is not
very one-sided and could get corrected in a day or two.