You may be using terminologies incorrectly. (I believe your question needs to be reworded.)
SQL is how you do CRUD activities with the database.
PL/SQL is the Procedural Language on top of SQL.
SQL*Developer is an application you use to write SQL and PL/SQL code (among other things)
SQL does not have APIs.
(There is the Oracle Data Cartridge Interface [ODCI], but that is more of an API used to enhance the database's capabilities.)
One method of understanding the RDBMS is to think of your Packages as APIs. These are written in PL/SQL (for Oracle)
That means, as a developer, you are responsible for developing the API yourself.
Oracle comes with a lot of Packages (APIs) ranging from Advance Queuing to Parallel Execution and beyond.
Each one has its purpose.