tablespaces and schemas

This post is as much about getting some questions answered as leaving
the following definitions in the archives for the next person.

After a quick perview of the web, I came up with the following:

tablespaces are a hardware issue, and totally transparent to SQL
execution. It is for optimization for IO, recovery, and separating user
and application usage amongst disks even in the same databases.

schemas are a logical issue, and NOT transparent to the SQL. If schemas
are involved, the SQL needs to know which schema tables are in to access
them.

My questions are:
1/ Am I right/
2/ is the use of the '.' character standard across all databases as
a schema delimiter, i.e. SELECT * FROM {schemaname.tablename.columnname;} ?
3/ Once a user/dba gets down to the actual SQL, and past all the
bl***ng Oracle Obfuscation(TM), does Oracle do the same thing with
schemas that Postgres does, i.e. the aforementioned '.' separator?

I am building an application that I want to work on Postgres, IBM DB2,
Oracle, MSSQL, et.al.

TIA, y'all.