Long, technical post ahead:
To understand the answers to some of your questions about throws, you need to have a rough idea of how invincibility works in this game. Simply put, there are 3 types of invincibility - hit, throw and projectile. A (part of a) move can have any of those attributes, or none of them. This also applies to character states (eg. grounded vs aerial). Just because part of a move is invincible to hits, it doesn’t mean it’s invincible to throws (or projectiles) and vice versa. eg. Blanka’s EX ball is invincible to projectiles and throws, but he can be hit out of it. Viper’s HP Thunder Knuckle has startup invincibility against hits, but she can be thrown out of it. The same goes for Juri’s EX Pin Wheel.
Grounded normal moves have no ground throw invincibility, which means that if a normal move and a throw “collide”, the throw will always win (provided that it’s in range, eg. you can’t perform a normal throw on Dhalsim by grabbing his outstretched limbs). When it comes to command throws, the general rule is that command throws beat normal throws, which means if these two collide, the command throw will win. Now as I said, it’s a general rule, but again it comes down to the what type of invincibility the command throw has. Eg. Abel’s EX Tornado Throw is a command grab, but it does not have throw invincibility, which means he can be thrown out of it.
(Note that it’s a bit more technical than this - you have to take into account startup and active frame invincibility - but that’s roughly how it works)
Grounded normal throws themselves have a startup of 3 frames and can be teched at any point from the 4th to the 10th frame (Gouken’s back throw is a special case, but it’s not important for this discussion). Throws and throw teching are also two of the highest priority moves in the game, meaning that if you input something that has a throw/throw tech command (lk + lp) the throw/tech takes precedence most other things if your character is in a state to perform it. eg. If, as Akuma, you do QCF + lk + lp on the ground, he will attempt a throw. If you do it in the air, he will perform an air fireball instead because he doesn’t have an air throw.
Armed with this information, it should be clear why mashing crouch tech is not really a good idea in most cases. If you happen to crouch tech slightly earlier, at the same time or even several frames later than the throw starts up, the throw will always win. Since you can only tech throws on or after the 3rd frame (I forget which), inputting crouch tech will not execute the tech because there is no throw to tech yet, so you get a normal move instead, and because normal moves don’t have throw invincibility, you end up getting thrown. Also, some characters’ pressure strings have slight gaps that allow you to perform a move, but the gap isn’t big enough to allow the move to connect, so the next move in the string counter-hit your character if you are mashing crouch tech or any other non-invincible move (this is typically known as a frame trap).
The preferred method of using crouch tech is using a technique called delayed crouch tech. Since throws basically have a 7 frame window to tech them, the idea is to try to tech as late as possible. This basically helps you defend against narrow frame traps (pressure strings with small gaps in them) and throws at the same time. Note though that there’s nothing stopping your opponent from delaying his attacks a bit longer to be able to hit/throw you out of even a delayed crouch tech.
To get a feel for the timing of a delayed crouch tech, have a Ken or Ryu dummy do neutral jump in front of you, and then throw you as soon as they land. Then in the same recording, let them do a neutral jump and but input this command :f::d::df: lk + lp. This will result in a dragon punch, or it will tech a throw if you try to throw them as soon as they land. To defend against this, use delayed crouch teching. In the first case, you’ll tech a throw, in the second case, you’ll block the Dragon Punch. If you are getting thrown, you’re teching too late. If you get hit by the Dragon Punch, you’re crouch teching too early. In practice, delayed crouch teching feels similar to tapping tech in slightly rhythm to your opponent’s block string, but just delayed a tiny bit. It’s also worth noting that the way to “react” to throws is to react to the opponent’s standing walking/animation (if they’re doing a low block string) and then preparing to use a delayed tech. It’s virtually impossible to consistently react to the actual throw animation itself.
You can also use the same training dummy technique for delayed standing tech. If the dummy ends up teching your throw in either case, you’re doing it too early. Note that there’s no real disadvantage to this in a match, except that if the opponent jumps lands out of range you’ll be left wide open for a punish when your throw whiffs (this includes them being able to counter-throw you because in many cases a whiffed throw increases the size of your throwable hitbox).
To effectively use standing tech, you have to know which of your opponent’s pressure strings don’t have low moves after the first attack. Basically, you perform a delayed crouch tech after the first attack and then immediately stand and tech. If they don’t throw, you’ll block the next move. If they do, you’ll tech it, and the big benefit is that even if you do it too early you won’t get thrown. Instead, they’ll end up teching your throw (just as with the training mode example). Another big plus is that if they are really close to you and there’s a gap between their attacks, you’ll throw them out of their next move (normal moves don’t have throw invincibility, remember?). Note that some characters have ways of beating this though, so take care when using it. Eg. After this technique was discovered, Japanese Ryu players started using cr.lk xx cr.lk as a block string starter instead of cr.lk xx cr.lp to catch opponents who tried to stand and tech after the first attack. Even so, standing tech is a big part of high level play especially against characters who can bully you with strong frame traps (eg. Dudley, Cody).