I am currently thinking of getting my first mountain bike, I have ridden a lot of cyclocross and am used to a 10kg bike but am not that familiar where you could save weight on a mountain bike.
What components would you use to reduce the weight of a 2018 Santa Cruz Chameleon which currently retails at 12.82 kg? I would mostly be riding rough cyclocross races and XC races. I would be purchasing a large frame size.
Breaking the rules with a kind-of non-answer and personal opinion, but...
1) Mountain bikes should be heavier than a cyclocross bike. It's got a suspension fork and frame and wheels need to be beefier to handle bigger loads and impacts.
2) Don't buy a bike with a plan replace major components. Just buy the bike that meets your needs in the first place (in this case a bike lighter than 12kg out of the box). Doing so will be much, much cheaper than swapping out parts.
Another way of stating (2) is: there is only one component you need to swap out for a lighter alternative - the bike itself.
I note the higher level SC Highball S hits your weight goal.
Bontrager's Law states parts can be light weight, durable, or inexpensive you only get to pick two characteristics. The only time it makes monetary sense to replace a part with a lighter one is if the component has failed. If you think 3kg. will make a difference between winning and losing try this experiment. Ride a timed course while carrying 2 full water bottles (approx. 1.5 kg) . Then ride the same timed course with out the bottles. Is the time difference greater than the time difference of the rider who finished before you. My point is that for the most elite athletes 3kg can be the difference between placing 1st or 15th. For most of us our times will vary more from conditioning variables than 3kg of weight.
To answer the question, you'd change the fork and wheels, probably the seat post. Beyond that, you work down the list of diminishing returns through group set, going tubeless, bars, removing rotor bolts (!!) etc. until you get to <10kg. This would take a lot of money for exotic parts.
However, I think you've chosen the wrong bike. The Chameleon is not necessarily designed for <10kg XC racing. As others have noted, you're better off looking at the Highball, which is more XC than aggressive/trail.
I own a 2018 chameleon. The frame is light. You could save weight on the wheelset with carbon hoops, lighter tires, lighter cassette. You could also get carbon bars and a carbon seatpost. It would be expensive and sort of silly.