I am not sure if there is a relational schema,
What we have done is created queries for each of the tables and then created joins within powerBI, bear in mind it only allows you to create one active relationship direct/indirect between tables , so you might have to create multiple measures to use the inactive relationships for specific queries.
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I do not know if there is an official document about relational schemas.
I shared on my drive a document that can help you a little. Please download and take a look, it is a doc of version 7.x, but can help.