All operations on a foreign table are handled through its foreign data wrapper, which consists of a set of functions that the core server calls. The foreign data wrapper is responsible for fetching data from the remote data source and returning it to the PostgreSQL executor. This chapter outlines how to write a new foreign data wrapper.
The foreign data wrappers included in the standard distribution are good references when trying to write your own. Look into the contrib/file_fdw subdirectory of the source tree. The CREATE FOREIGN DATA WRAPPER reference page also has some useful details.
Note: The SQL standard specifies an interface for writing foreign data wrappers. However, PostgreSQL does not implement that API, because the effort to accommodate it into PostgreSQL would be large, and the standard API hasn't gained wide adoption anyway.