AVIF supports a higher resolution at 2^32-1 pixels per dimension, vs JPEG-XL's 2^30-1 pixels. But really it doesn't matter since something is going to impose an arbitrarily smaller limit on either before letting you try to allocate more RAM than any computer can address. Yes, plenty of comparison websites confuse AV1 limits (sometimes AV1 profile limits!) with AVIF limits.

For arbitrary planes, AVIF calls them auxiliary images. Unless you mean encoding non-RGB data to start with like how DNG uses JPEG-XL. At which point you're outside the scope of any standard interpretation of JPEG-XL data.