All rechargeable batteries have a finite usable life. Battery capacity will be reduced, ever so slightly, for every recharge, this degradation being cumulative.
An iPad battery is rated to maintain at least 80% of its initial capacity after 1000 full charge/discharge cycles - whereas an iPhone is rated to have 80% of initial capacity after 500 full cycles. A full cycle is defined as any combination of charging cycles that together total 100% (e.g., 50% + 20% + 20% = 100%).
There are software packages available that can access battery cycle count. Popular choices for Mac are Coconut Battery and iMazing - whereas iMazing is available for PC.
https://www.coconut-flavour.com/coconutbattery/
https://imazing.com/