Compare the tablespace concept between Postgres and Oracle When I create a table, it is placed in a default tablespace such as "USERS", unless I specify a user created tablespace, say, MyTableSpace.

For example,

create table test_tab(

name varchar2(100),
photo BLOB,
text   CLOB
)
tablespace "USERS"   --default
lob ("photo")  tablespace "MyTableSpace"   --user created
lob ("text")  tablespace "MyTableSpace"    --user created


Now, convert the above table to the counterpart in Postgres.  Where could those columns be placed?

A default tablespace?  User create tablespace?  Or some proprietary logical structure in Postgres?
In PostgreSQL, if you don't specify a tablespace in your CREATE TABLE command, the value of parameter default_tablespace will be used.

If default_tablespace is not set, or has an invalid value, the table will be created in the same tablespace as the database.

https://demo.pganalyze.com/databases/6/config/default_tablespace