Do we use shared_buffers for WAL ?

No.

What's about the explanation in https://www.postgresql.org/docs/12/runtime-config-wal.html : "wal_buffers (integer)    The amount of shared memory used for WAL data that has not yet been written to disk. The default setting of -1 selects a size equal to 1/32nd (about 3%) of shared_buffers, ... " ? My understanding was, that the parameter wal_buffers grabs some of the existing shared_buffers for its own purpose. Is this a misinterpretation? Are shared_buffers and wal_buffers two different shared memory areas?

Kind regards, Jürgen