The `app-portage/bashrcng-patching' Gentoo package must be installed.
It used to be in overlay :: layman :: gechi... Yet, the download works no longer...
So, I have old copies of their eselect scripts, when `ebuild'em anew in phases.

If the plugin is enabled, one gets something like this.

user@localhost ~ $ eselect bashrcng-patching show
Actual plugin settings:
  patch(es) overlay: /usr/portage/local/patches

The patch(es) is/are applied for an .ebuild prior to the `prepare' ebuild phase.

In the case of an EAPI without support for `src_prepare', packages
must be patched via user intervention, upon `ebuild ... unpack' end.

To eye-catch the output whether packages are patched
automatically, emerge with option "--quiet-build=n".

The patches come under a `<category>/<package>' directory for every package.

But each `<category>/<package>' resides under a special directory "repo",
that actually contains the patches classified by overlays / repositories.

For example, the main portage tree is called "gentoo".
This eases the work with overlays.

Whilst all `<category>' directories are real,
each `<package>' subdirectory is a symlink
pointing to its `repo/<category>/<package>'.

It could also be that `<category>/<package>' is
a real directory too, but then each patch must be
a symlink, possibly pointing within different repos
corresponding to disjoint sets of versions.