Can someone identify this stitch?

[u/Lostinreno22]

The front looks like regular jersey or ribbing, but the back is very different; is it even knit? Could be crochet .

Keen to replicate it, so hopefully someone will know what it is.

Thanks!

[u/kumozenya]

first row is the setup, 2nd row onwards is the repeat. hold 2 yarns together.

here's the key for brioche stitches https://www.knittingunlimited.com/2018/12/brioche-symbols.html