How can I get accurate information about disk space used/available in Big Sur via the command line?
diskutil info /dev/disk3s1s1 seems to do the trick for me:
Container Total Space:     994.7 GB (994662584320 Bytes) (exactly 1942700360 512-Byte-Units)
Container Free Space:      635.1 GB (635058384896 Bytes) (exactly 1240348408 512-Byte-Units)

(other output snipped)
My output from df -H:
/dev/disk3s1s1 995G    15G   634G     3%    568975  9712932825    0%   /
/dev/disk3s5   995G   335G   634G    35%   1363090  9712138710    0%   /System/Volumes/Data