Very broad question gets a very broad answer. It's machine specific.

Biggest bonus points come from battling the power/powerfield and powerball jackpot.

Other bonus from features in the game that don't regularly score points during play. Things that build up to other progressions. For example, the Hitchhiker, doesn't collect until the bonus or until Hitchhiker is lit from the door. I guess that's the best answer I can give. Hope it helps.

On modern Sterns the number appears to be "magic mixed with math". I should just do calculated numbers of switch hits and drain to see if some patter emerges (e.g. slings add X, but loop switches add Y, etc...).

It is a bit of a shame compared to old games where going after bonuses was a big part of the strategy. Now, save for the odd game with a broken bonus system (usually abetted by huge multipliers) bonus is pretty irrelevant to scoring strategies on modern pins.

Yes, much easier to understand on EM and early SS games. And bonus was usually much more important in these games - often requiring a shifting strategy of going for bonus or going for multipliers. later SS and particularly DMD usually have various jackpots that are far more important so bonus is often an afterthought. MM is a great game, but I pretty much never care if I tilt, since saving the ball is almost always worth the risk, since any ball that earns a meaningful bonus is already going to have a ton of scoring behind it. On most DMD games it will give you a quick rundown of the various sources of bonus at the end of ball and then total everything up and multiply.

No. Defeat the Power and Powerfield increases Defeat the Power total. But Powerfield and Powerball Jackpots are awarded at the bonus. And TZ does have a Collect Bonus feature on the Camera. Same with Hitchhiker until collected on the Door, but will award at the bonus whether collected or not.

That's the point. You got to know the game to know where the big bonus points are. Some games, bonus isn't worth worrying about.

In most DMD games the bonus value is, as said above, magic mixed with math.

On sterns it's usually something like

125k * ball in play + X * switches hit + Y * some subset of switches hit (pops or slingshots or whatever)

Where X and Y depend on the game.

The short answer is "it depends, and it's usually not worth caring about".

Other games have very specific bonus counts: in Tron it depends solely on how many of the lights on the ladder are on or blinking. In WOz a large part of the bonus is how many Yellow Brick Road shots you've made. In AC/DC it's a count of your 'song requests' and songs played.

Many Bally/Williams games from the 90s included some or all of the points from various things (modes, especially) in the bonus. Why? Who knows. Started with Addams Family. Generally these points are not_ multiplied by the bonus multiplier, but are added after the multiplication. This was popular through at least Shadow, where your bonus was often half your score.