Anyone with 7zip could do it in seconds
Edit: didn't preview the video, yes, aware he didn't use 7zip in this one. Still.
I didn't say it was hard, I said it was above your paygrade. That's a DOS command, after all.
Also, concatenating a JPEG with a RAR is pretty amateur and sounds like something a lazy fat fuck would do. That the resulting file opens correctly in both Photo Viewer and WinRAR is because the JPEG parser stops reading the file at the JPEG-specific EOF per the spec, and the RAR parser is looking for the RAR header and ignores everything before that, reads up to it's format-specific EOF, and ignores everything after.
My argument about pedophiles and plausable deniability stands. Having a CP-containing RAR that looks like an innocuous JPEG doesn't get you out of jail, but it might get you distribution charges.
BTW it doesn't work with the RAR first and JPEG second, because JPEG parsers expect the JPEG magic number to be at the beginning of the file.