During an ongoing phone call, there are two virtual buttons on the screen: the Bluetooth button and the loudspeaker button. There is a line between them to visualise that they belong together - they are audio-related.
This is how it works:
a) Bluetooth button is lit: call audio is routed to the connected Bluetooth device
b) Loudpseaker button is lit: call audio can be heard from the handsfree loudspeaker of the phone
c) Neither of the two is lit: call audio goes to the earpiece.
Attah, you should use option c).
For comparison, Samsung Galaxy S9 (Android 10) has a similar arrangement.