Blocking standards are fairly reversed between 3D and 2D games. Blocking low and watching for overheads is the standard blocking tactic, as low attacks are typically fast and safe, and mid attacks can be blocked high or low (most attacks tend to be mid attacks in general). Overhead attacks are typically less common and slower/risky/obvious, but may lead to higher damage (quick lows generally lead to less damage). If the opponent jumps you generally block high since most jumping attacks hit overhead. You also have the option of jumping and then instead of attacking high you can land and go for a low attack, generally this is called an empty jump mixup.
You cannot duck under throws. The most common ways to avoid throws are to jump, tech the throw, or perform an invincible move (reversal). Other methods can include backdashing, jabbing them out of throw startup with a fast normal, or performing an attack that is throw invulnerable or puts you in an airborne state, but these are more game dependent and not universal rules for every game.
The basic mixup in a 2D fighter is hit/throw (or low/throw). The idea being that when an opponent is blocking low (the “default” defensive posture) you can either hit them with an attack (often a low attack like a crouching kick or something) or you can grab them. If they think you are going to grab them and try to jump but you chose to attack, your attack will hit them during their jump startup frames. If they try to tech the grab (usually by pressing the grab input) but you attacked, your attack will hit them out of grab startup or the startup of whatever move might come out when you miss a grab attempt (this can vary by game). If they choose to block and you chose to grab, you will throw them.
Of course there are plenty of other mixup tactics like overheads, crossups, unblockable attacks, etc., but the strike/throw mixup is the most basic tool that every character always has
edit: to discuss more on jumping, its usefulness varies by game. In a game like street fighter where most characters have minimal options for either defense or mobility in the air, jumping is risky and much less common for most characters. In other games there may be a variety of options for defense and mobility during a jump, in which case it may be more desirable to do so. As a general rule, jumping makes you vulnerable to attacks from the ground that specifically target the vertical space (anti-air attacks). The main benefit of jumping is usually that it puts you up in the air, and a forward jump often covers more distance faster than walking or running might.
Consider this situation: you and the opponent have some distance between you. You believe he is about to throw a projectile that travels horizontally. If you jump forward, you will jump over the projectile and may be able to hit the opponent on the way down as they are still recovering from the projectile-throwing animation. However, If they did not throw the projectile, they may be able to easily hit you out of the air with an upward-striking attack while you are vulnerable. This is a very basic example, but jumping can be a very important aspect of movement in 2D fighters.