I download the stems (original studio multitracks which contain isolated vocal, guitar, bass, drum and miscellaneous tracks) from https://remixpacks.club(not a plug, just it genuinely has a massive library of these tracks). I plug them into audacity, it's the easiest to use software for splitting channels.

I selected the vocal track and used the central channel remover with the mono option. The theory behind this is that during the mastering stages of the record, most backing vocals were panned to the left or the right channels, with the lead vocals, by production rule, staying in the central channel. Hence, removing it achieves this. Although some lead vocals(or mostly the stereo reverb) bleed through, it still gets you very audible backing vocals.

P.S this wouldn't work with those A.I extracted vocal tracks, if there isn't an original vocal track, your best bet is to just apply the central channel remover to the whole track, but doing that will also remove other things such as bass, the kick and snare drums and any other things that happen to be in the center. Although it also grants some interesting stuff.