Doesn’t charging iPhone 8 Plus with 20w fast charger drain it’s battery life ?

Fast charging does not negatively affect the battery.

Manvitha_reddy wrote:

I just want to know whether it drains the battery in the eventual time or not

No. Batteries in iPhone are smart enough to shut off when full.

All batteries age.

You have already had this question answered in another post.

Repeating it will not alter the answer.