Someone, @ldir, in another thread shared a link to what looks like an excellet write-up on the connection tracking aspect of nft. A 3 part series so far.
Link to that thread post:
https://forum.openwrt.org/t/nftables-and-qos-in-2021/112013/565
Quote from that post:
https://thermalcircle.de/doku.php?id=blog:linux:connection_tracking_1_modules_and_hooks goes into more depth than my mind can cope with.