iFixit has separate guides for the charging-case cell and each earbud cell. The earbud guide treats left and right buds as symmetrical for battery service.
What to check before buying
Earbud batteries
The iFixit guide replaces one WF-1000XM4 earbud cell and says the same steps apply to the other side. An overheated bud or incomplete charge may need earbud-level work rather than a case battery.
Charging case
A separate guide covers the case battery when the case fails to hold charge or its status LED behaves unexpectedly. Distinguishing case and earbud symptoms matters before opening either small enclosure.