why my battery health is going down so fast

That value is approximate and, most of all, it is related to when the battery (not the iPhone) was brand new.

iPhone 12 series was firstly released in September 2020, and if you bought that device second hand, it might be a normal behavior.

For your piece of mind, I suggest that you make an appointment (*) and have your device inspected at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> Apple – Location Selector. After checking it out, they'll give you all the available options.

Regards

Giulio

(*) You can do it:

• Online:--> Genius Bar Reservation and Apple Support Options - Apple;
• Contacting a local Apple Store or AASP among the list provided above;
• Using the "Apple Store" app, available and downloadable from AppStore.