when my battery on 7a reaches 15 percent or so.. it drains instantly

lovefromisrael what is your Settings > About phone > Battery information > Cycle count?

You might need to read this.
https://makingcircuits.com/blog/how-to-charge-a-li-ion-battery-correctly-and-safely/

Do you charge it to 100% often? You need to do that sometimes to recallibrate the capacity.

lovefromisrael OK, but don't expect me to précis it for you.

lovefromisrael strange. Then i have no ideas.

lovefromisrael do i have to hand my password when they replace the battery?

It depends on who is doing the replacement. I have a faint sense that Google's standard workflow involves running diagnostics on the device, which might result in it being wiped, so I would definitely recommend that you back up whatever you can.

lovefromisrael should i ask for an original battery for replacement?

That's up to you. I think I would.

lovefromisrael ive now noticed that the battery got bloated

I got bloated battery on my 2013 MacBook Pro every 3 to 4 years because I always left it plugged in. Last year I installed Al Dente which can cap the charge at 70% (very similar to the 80% charging optimization in GOS) so I don't have to worry about the bloated battery anymore.

I also learned that from 80% to 100% is the most damaging phase when charging the battery so need to avoid that as much as possible. Your 15% problem might well be a calibration issue - you need to drain the battery to nearly empty then charge it to 100% in order to calibrate. But that's a moot point now that your battery is already bloated and needs a replacement.

lovefromisrael I think Google has an online tool for calibrating fingerprint sensors after screen replacement.