The shim_et/ subtree helps the Meta-internal buck2 build system also work in the open-source repo.

Shims are how open-source buck2 supports a line like:

load("@fbsource//xplat/executorch/build:runtime_wrapper.bzl", "runtime")

In the open-source repo, fbsource//xplat (a Meta-internal root) doesn't exist. The fbsource = shim_et line in ../.buckconfig tells buck2 to look in shim_et/xplat/executorch/build/runtime_wrapper.bzl instead.

NOTE: tools is a symlink to ../shim/tools because fbsource// must refer here, but fbsource//tools in particular lives in shim/.