If they’re using hit scan, sometimes you’ll see them shooting automatically even when the opponent is around a corner.
If they’re using wall hacks, you can see them knowing exactly where everyone is at all times.
Sometimes the movement is akin to a bot the way they turn/ snap to aim especially if you feel like they won a duel against you where you had the upper hand.
Sometimes anyone can hit melee mid burst of a battle rifle. This interrupts the burst and makes the next burst fire 4 rounds instead of 3 (you can see people doing this quite often in the HCL professional series, the tell is when one of them have an ammo counter that is not divisable by 3) - But this is more of a bug/ exploit than a cheat.
There’s also rapid fire for the magnum and BR that can be activated using chronos. A third party piece of hardware you can get on ebay for $70 or so. It plugs in between your console and controller letting you speed up button sensitivies and such- people also use it to play on xbox with a ps5 controller etc.
If you are this interested in what cheats are out there- why not just youtube search some footage of what the other 20 million players etc have encountered. I’d say you got real lucky getting this far without running into one. There’s plenty of PC cheat sites offering various chats and hacks as well, just have a quick google, sign up and see whats available.
Without significant anti cheat, and making a blog post noting there is no anti cheat, 343i kind invited cheaters into the game. More so in larger numbers than any prior halo had.