how do i retrieve property a of a pulumi resource using python?

You can’t print an Output because an output is a container for a deferred value that is not yet available when print is called. Instead, try exporting the value as a stack output

pulumi.export("destination", destination)

If you really want to print it, try doing so from within an apply:

destination.apply(lambda v: print(v))

By the way, your first snippet can be simplified to

destination = bucket.id.apply(lambda id: f"storage.googleapis.com/{id}")

but concat is even simpler indeed.