Why is my battery draining so fast?

A four year old battery may well be at the end of its life, so it may simply be time to replace it.

Think of the amount of charge a battery can hold like a jar you can fill with water. When the battery is new, you can fill it with water all the way to the top. Now each time you use it, you add a marble. That reduces the amount of water (charge) it can hold by a tiny fraction, but over time after you've added enough marbles, even though the jar is full (your battery is charged to 100%), it is only holding a fraction of the charge it could originally. So when you start pouring out the water - using the battery - you don't get nearly as much as you did when it was new, so it only lasts a short amount of time before it has to be refilled.

This YouTube video shows you the basic steps you need to follow to replace your battery.

Realme C21Y Battery Replacement - Easy Repair Process@realmemobiles - YouTube

My main complaint about this video is that there's not that much detail, although it is pretty much complete it glosses over the screws you need to remove and the actual removal of the battery. The following video is much better in those regards.

Realme C21y Open / Battery Remove - YouTube

The issue I have with this one is that the author removes several parts that don't appear to be necessary just to get the battery out. I would definitely not bother unplugging the connectors on the motherboard and removing it; all you need is to take off the cover and unplug the battery. Likewise, I see no reason for removing the lower cover or any parts below the battery whatsoever.

I'd say use the first video as a guide to what steps you need to take, and the second one for details on exactly how to do those steps.