I will refer to AD&D1e because some of the later games have tried to copy ideas from it, but the writers don't necessarily get how things go.
(1) in AD&D1e, having sufficient XP qualifies you for the next level, but until you are the next level you cannot receive any more XP. You can think of it less as total XP, and more as "I have earned this many XP at my current level." It's like how it doesn't matter how well you performed in Calculus 101, you still have to do Calculus 201 before they'll let you do 300-level maths. Likewise, you can't skip from 1st to 3rd level without spending some time adventuring at 2nd. That's why AD&D1e has training to level up.
The DM rates the players' roleplaying (did the fighter fight, did the thief grab extra opportunities for loot, did people follow their alignments, etc) 1-4. The cost of levelling up is 1,500gp x the current level x rating, and takes as many weeks. For example, a fighter fought boldly in the front line and is rated 1 by the DM. The are currently 2nd level, and the various monsters and treasure found accumulate to enough to more than pass into 3rd level. They will take (current level) 2 x (rating) 1 x 1 weeks = 2 weeks for them to level up, and cost 3,000gp.
If this same fighter had lurked around at the back, not coming forward even when the magic-user was stuck in melee, and waiting until the monster had 1 hit point left before striking, then they would be a poor fighter. Because they've not acted like a fighter they have more to learn than the other guy did. So the DM rates them as 3 in performance. Now it takes them 6 weeks to level up, and costs 9,000gp. It may be that the character has the XP to level up, but not the gold. Time to keep adventuring! Or it may be that the rest of the party who played well finished their training a month ago and have gone and had another adventure sufficient to qualify them for 3rd level - while you're still training.
In either case, now being 2nd level, they are now in a position to learn the things required to get them to 3rd level.
(2) in AD&D1e there are some rules of gp for XP being modified by relative level - a 1st level magic-user dealing with 10 kobolds is one thing, a 10th level magic-user dealing with 10 kobolds is another. Which is to say, you learn from struggle, less struggle, less learning. Thus, paying other people to go adventuring for you, or buying and selling things as a businessperson, will get you ZERO XP. You'll still have the gold, but no XP from it.
(3) in AD&D1e there'll be movement rates for various characters based on their size (a man walks further than a hobbit) and encumbrance (someone in leather moves faster than someone in plate). For my part, I rarely keep track of it except to say that if they faff about too long then I'll do a wandering monster check.