The Metadata Working Group regions tag (XMP-MWG) is the defacto standard for storing face regions. This is where most modern software stores the data whether it is embedded in the file or in an XMP sidecar file. While the MWG is defunct, you can read the standard through Archive.org.

Microsoft created a region tag (XMP-MP) to work with their Windows Live Photo Gallery program. I think OneDrive uses this, but I don't think I've seen anything else use it.

The IPTC Photo Metadata Standard included a region tag (XMP-iptcExt) as part of their spec a few years ago (2019). This is still pretty recent and I haven't seen software support for this yet.

ACDSee, as they so often do, went and created they're own standard for face regions (see the author of exiftool Phil Harvey's comment on this) . It is, for the most part, a copy/paste of the MWG standard. It differs in the fact that it saves regions recognized by the ACDSee program separate from faces manually applied. ACDSee will read MWG regions and convert them to ACDSee regions.

As you can see, all of these regions are stored in XMP. That is because XMP is far more flexible than the EXIF standard is.