when asking that question : what tables and how many rows are affected by the actions in a single wal file
I was directed (correctly) to using pg_waldump and treating the zzz in rel xxx/yyy/zzz as the relfilenode which identified the table (or more accurately: the relation) which was affected by an entry in wal files.
I tried reading the documentation for pg_waldump and wasn't able to find a description of the output format.
By similarity, I was able to understand that the xxx/yyy/zzz in the output matched the format for the --relation= format (and is probably namespace oid/database oid/relfilenod), but I did not find on this page a sentence stating "in the output, rel xxx/yyy/zzz refers to ...".
For example: I don't know how to make sense of the len (rec/tot): 2063/ 2063 field.
Question
Where can I find a description of the output format of pg_waldump ?