missing something about json syntax

postgres 15

looks Iike I am missing something, maybe obvious :-(

In a table with a json column (_data) if I ask psql to select _data from mytable with a where clause to get only one line,, I get something beginning by

 {"time":"2023-04-19T16:28:01.19780551+02:00","stream":"stderr","_p":"F","log":"{\"level\":\"info\",\"ts\":\"2023-04-19T14:28:01Z\",\"logger\":\"_audit\",\"msg\":\"record\",\"logging_pod\":\"cluster-pgsql\",\"record\":{\"log_time\":\"2023-04-19 14:28:01.197 UTC\",\

etc...

if I create table anothertable as select _data as _data  from mytable, it creates and feed that new table with all the appropriate data, and when I ask psql \d anothertable it says that its a table with a json column.named _data.

fine !

now if I select json_object_keys(_data) from mytable, I get a list of tags. time, stream, _p, log, fine.

now, if i select json_object_keys(_data) from anothettable, I get an error: cannot call json_objet_keys on a scalar..

???

both columns are fed and of type json. and postgres didn't throw any error feeding them.

if I create a table with a jsonb column and feed it with the anothertable json column, same, fine... but still unusable.

and unusable with all the other ways I did try, like simply select _data->'log'->>'level' from mytable, or select _data->'level' from anothertable

sure if I look at the json field one is showed { "tag": "value", ...

and the other is showed "{\"tag\":\"value\", ...

not the very same

so 2 questions:

1) how postgres can feed a json or jsonb column and CANNOT use the values in it ??

2) how to "transform" the inappropriate json into a usable one ?

of course, if what I am missing is very obvious, I apologize...