Can someone point me to any doc describing why and how much space postgres uses on the swap of a debian machine ?
it's an old postgres 10, because it is used by a product for which only this version is certified.
(no comment on that, please)
Swap as in RAM swap space? If so, then it uses as much or as little as Linux thinks needs to be used, given the amount of RAM.
effective_cache_size, work_mem, shared_buffers, maintenance_work_mem, max_worker_* and max_parallel_* affect how much RAM PG uses, in conjunction with the number of active connections (including maintenance work).
