When yank-indent-mode is enabled, yanked (pasted) text is indented based
on the indent rules of the current major mode. It has a
global-yank-indent-mode too which by default excludes a long list of
known indentation sensitive modes with which this approach does not work
very well.
It is based on some random hacky snippets elisp I've been using for over
a decade. Said snippets are themselves based on some random snippets I
found online, and since morphed into the weird monster they had become.