Concurrency Question

I'm trying to clearly understand how foreign key constraints work. I
still need some help.

The PostgreSQL documentation says:

> ROW EXCLUSIVE
> Conflicts with the SHARE, SHARE ROW EXCLUSIVE, EXCLUSIVE, and
> ACCESS EXCLUSIVE lock
> modes.
> The commands UPDATE, DELETE, and INSERT acquire this lock mode on
> the target table (in addition
> to ACCESS SHARE locks on any other referenced tables). In general,
> this lock mode will be acquired
> by any command that modifies the data in a table.

So if my foreign key constraint is: table A b_id references b(id)

and if table B already has an try for id = 5 and I do an insert into
table A with b_id of 5 how does the database ensure that the entry in
table B will still be there by the time the transaction ends? e.g.
if there is an insert into A and a delete from b of id = 5, if the
delete happens first, then the insert should fail. If the insert
happens first, then the delete should fail. But how is this
accomplished?

Looking at the documentation above, I would expect the insert into A
to get a Row exclusive lock for table A. And, I'm guessing it would
get an ACCESS SHARE lock for table B. But this would not prevent the
delete from B from happening at the same time (if I am reading this
correctly).

Can someone help me out here?

Thank you,
Perry