Relational database vs object-relational database

Question

There's plenty of questions on SO about the differences between OO databases like MongoDB vs relational databases, but I can't find anything about the differences between a relational database like MySQL vs an object-relational database like PostgreSQL at a high level. I understand that these two particular databases have differing functionality respectively, but I don't really understand why something like MySQL that claims to be simply relational doesn't return the same formed data as an object-relational database like PostgreSQL. Some of what I read on other sources say it's what they return when queried that is different; is this true?

Answer 1

Maybe this entry from DBA Stackexchange would be helpful:

https://dba.stackexchange.com/questions/77340/how-to-understand-object-relational

In particular, this answer: https://dba.stackexchange.com/a/77344/111252

Essentially it describes built-in inheritance features of Postgres that are outside of the SQL standard. That seems to be the key difference.