The GitHub docs page for CODEOWNERS is not very helpful in terms of how the file is interpreted. I’ve done a little experimentation to figure out how it works, and here are the results.

Rules

For each modified file in a PR, GitHub examines the codeowners file and selects the last matching entry. It then combines the set of mentions for all files in the PR and assigns them as reviewers.

An entry can specify no reviewers by containing only a pattern and no mentions.

Test

Consider this CODEOWNERS:

*            @org/reviewers
*.js         @org/js-reviewers
*.go         @org/go-reviewers
security/**  @org/sec-reviewers
generated/**

Then a change to:

• README.md would get review from @org/reviewers
• src/foo.js would get review from @org/js-reviewers
• bar.go would get review from @org/go-reviewers
• security/crypto.go would get review from @org/sec-reviewers (but not @org/go-reviewers!)
• generated/reference.go would get review from nobody

And thus a PR with, for example:

M src/foo.js
M security/crypto.go
M generated/reference.go

would get reviewed by @org/js-reviewers and @org/sec-reviewers.

If I wanted per-language reviews even under security/, then I’d use

security/**       @org/sec-reviewers
security/**/*.js  @org/sec-reviewers @org/js-reviewers
security/**/*.go  @org/sec-reviewers @org/go-reviewers

http://code.v.igoro.us/posts/2019/07/codeowners.html