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

Fast charging does not negatively affect the battery.

You have already had this question answered in another post.

Repeating it will not alter the answer.

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.