How is the Left / Right terminology for Relationships decided in BaseElements

Actually, BaseElements doesn't decide which is which. This is decided by FileMaker and is hard coded into the Database Design Report.

The actual decision is made at the time that you create the relationship. If you're creating it via the "New Relationship" button, you're choosing a left and a right. If you're using drag and drop, then you're actually having FileMaker decide for you in the background. Either way, once that decision is made, it remains that way, and altering the layout of the graph doesn't alter the output of the DDR.

Which leads to the other question : How do I see all TOs or all fields used in both sides of a relationship?

In terms of referencing the TOs or fields, switch to the Items Used tab, and use the portal there. This includes TOs and fields used on both sides, and in sorts as well.

If you're doing a find based on a one sided value such as the "delete related records" option, you will need to add a second find criteria to your search so that you find once for each side of the relationship.