I personally think it’s a weird fudge… but general advice is to add BOTH content blocks to the template.
Add POST CONTENT → Source → Wordpress
Add AT ROOT LEVEL - under this - POST CONTENT → SOURCE → BRICKS
It will populate from whichever is used. I don’t like it, because you could have a client ‘classic edit’ a BRICKS page, add their stuff to classic, and then it renders both.
I’m a little baffled as to why this fundamental feature is so ham-fisted in an otherwise exceptional product. I absolutely agree there should be a simple selector in the page editor to say ‘this template’.
My solution is to force people to use Bricks for pages, but lock the editor down a bit. Bricksforge is good for this - I use AdvancedThemer, because 99% of my clients don’t edit their own pages - only posts - so ‘blanket lock down’ is sufficient.