The small spot behind the Bluetooth ear case is the charging hole, which is used to charge the ear case. This charging hole is usually sealed with a rubber plug to keep out dust and other debris. When charging the ear housing, you need to pull out this rubber plug and insert the charging cable into the charging hole.
After charging is complete, the rubber plug needs to be re-inserted into the charging hole to prevent the entry of dust and other debris.
Some Bluetooth earcases also support wireless charging, so there is no need to use a charging cable to charge, just place the earcase on a compatible wireless charger.