Is it learn how to fight by watching other people fight ? Or by fighting other people..?
There are of course some things a person can learn from watching another and actually that is how some people best learn is by observation. Others learn by explanation and doing while others learn by a combination of both of those two approaches to learning. However while there are some things a person can learn about fighting and watching others there are a whole lot of other things that they have to instead be taught and coached on as well as practice doing themselves to learn and that especially applies to something like fighting. Some aspects of fighting actually run contrary with a person's natural reactions and they having to learn how to overcome those natural reactions. Flinching for instance is a good example of what I am saying and how a person needs to learn to overcome that natural reaction if they are going to be effective and a good fighter. Backing up while dropping or lowering your hands and putting your hands more out to the sides (to catch yourself in case you trip) is another natural reaction and example of what I am saying. So while you can learn some aspects by watching others there are way more that are not so apparent that need to be taught, coached, and learned.
I do not think so
it depends what your goals are if u want to take a piece of the crimson pie then all bets are off
its possible to be really good at striking without any training just by fighting. but people who achieve this only get good cause they fought a lot as kids. its safer to fight when your a kid because theres less chance of injuries. so you can get a lot of experience without getting hurt. youll only be expert amateur at best using this method but thats still pretty good. but you need instruction for the other aspects of fighting. the normal way to get gud is to do drills and spar a lot. drills are important cause you build muscle memory of what to do. drills dont just help you learn they also help eliminate bad habits. if you only spar or fight you will learn bad habits that can and will be exploited at the pro level but probably not at the semi pro level. of course you also have to train physically but i left this aspect out because i felt it wasnt relevant to the question. and theres also just plain theory like having a game plan and understanding why and how you should fights specific people but this is usually handled by the coach.