Upgrading keycloak

Hi, I’m new to keycloak, so forgive my beginner question.

I am in the process to upgrading our keycloak server, from 15.0.2 to 16.1.1, from the documentation I can read, security realms are no longer supported, so my question is what am I supposed to replace security realms with ?
Also how am I supposed to update standalone.xml ?
I can see many differences there between the keycloak versions (15.0.2 and 16.1.1), but also between our version and 16.1.1 version ?
I can see there are CLI commands, but if I understand correctly those commands are for already running servers (I don’t even know how to invoke them).