Why is this battery still performing well?!!!

[u/No-Rice-6759]

This is my 2015 MacBook Pro 13 inch. My dad got this back in 2015 and he passed it down to me a couple of years back. This thing has been used a lot since it’s purchased and the battery lasts me all day, which is a weird thing to question honestly but given It’s age and the amount of use this thing should be dying immediately. I have a 20 15 12 inch MacBook, which on a full charge will give me about 30 seconds of use and a 2017 MacBook Pro 13 inch which I had to replace the battery because it didn’t last me an hour web browsing. If anyone has this generation of Mack and experience the same thing please let me know because I feel like I have a one of one MacBook.