So the first file acts as a sort of index? From the earlier comment I thought it was autodetecting the presence of the numbered files and expanding what it found.

[–] amelore@slrpnk.net 2 points 5 months ago

It's going to have some metadata to that effect yes, like a file index or number of parts or total extracted file size. I don't know the details, I've used them I haven't read the spec. rar is Rarlab's proprietary format so there might not even be a public spec.
They're normally all the same size except for the last part, so it's not that file 1 is just an index.