I submerged my iphone 7 plus in a pool and it start glitching after some time and now it is completely off. what do i do?

Do not attempt to charge it.

Make an appointment at the nearest Apple location to have your iPhone evaluated for free.

Once the diagnostics are complete, the staff will explain your options.

Make a Genius Bar Reservation

https://www.apple.com/retail/geniusbar/

Most likely dead now. When there's liquid damage if can start slow and then get worse since there's likely corrosion that just creeps further along.

What do you need from it? If you've got a decent backup and can recover from that, Apple's only solution would be and exchange of a replacement device. For an iPhone 7 Plus that's $349 plus tax in the United States. I would personally get a new SE for just a bit more.