Almost any modern programming language can open multiple files.

At a minimum it is multiple file formats. But really it is multiple file formats + a fairly intricate library for dealing with them along with locks etc.

According to their home page the SQLite database file format is a file format, and SQLite is a library.

No but close. A SQlite file is a single format. The journal file and the WAL file are different formats used for bookkeeping by the library to attempt to be ACID compliant. The library implements some complex logic to ensure this, but reading/writing a SQLite file does not require any of this.

You could write your own code tomorrow that reads/writes SQLite files but does not produce, read, or write WAL or Journal files. So long as the resultant SQLite file follows the specification, it can be read by any other piece of software that implements the specification, such as the SQLite library or SQLJet (https://sqljet.com/).