Update a connection field based on a relationship in another connection field / pre-populate connection field


I have an object with two connections. I want to update only one and the other gets populated automatically based on one.

When I try to add 'Conditional Rules' for one of my connected fields and set value, I can only see the same field name.

Same idea with setting up security between three tables. How to automatically prepopulated the connection field!

Please advise!