CoreBOSBB

When user click on delete on Opportunity for example, system remove all registry in relation too?

If I return a opportunity in this case, system will return a contact in relation too?

If exists any document associate a this contact will be remove too?

In general, any delete action will only affect the record being deleted, no related records are deleted. In most cases the relations are saved and restored when the record is recovered from the recycle bin, but I am aware of some cases where this does not happen correctly (I think in products).

So, no, documents will not be deleted. In theory you could create a Delete Related Records task on the Delete trigger to accomplish this though.