How to format a fixed-length number as a binary string?

I'm trying to wrap my head around how Postgres represents numbers as binary. How would I format a fixed-length number as a string of ones and zeroes? Example:

select fmt((1<<0)::int2); -- something like 0000 0000 0000 0001
select fmt((1<<2)::int2); -- something like 0000 0000 0000 0100

EDIT: I'm working on postgres 14.