We assume that you are asking about Battery Health / Maximum Capacity. Battery Health always decline with age. It never goes back up....unless....you replace the battery.
Battery Health on average declines at a rate of about 1% a month. If you bought the iPhone 12 when it came out, the phone is now about 3 years or 36 months old.
You would expect Battery Health to be in a range of around 65-70% by now.
Your battery is at 85%. That is amazing. There are a lot of users who would love to have the luck that you have had.
My iPhone 12 performed at about average. At 2 years, the Battery Health was about 80%. Normal and expected.
Your phone will need a new battery when it is down to 80%.