In dnd 5th edition, do my modifiers go up when I level up and how and why?
Most of the numbers on your character sheet either have, or can have an upward trend to them. However when and how often these numbers advance depends on which modifier you're talking about.
Your character's primary modifier is his Proficiency Bonus. This begins at +2 at level 1, and maxes out at +6 at level 17. It's progress is indicated by the "Proficiency Bonus" column in your class's table.
Your ability modifiers (e.g. Strength Bonus, Dexterity Bonus, and so on) increase when your ability scores increase. The main way that your ability scores increase is through the Ability Score Improvement feature listed in your class's table.
Whenever you receive an Ability Score Improvement, you have a choice between three options:
Increase one score by two.
Increase two scores by one.
Gain a feat.
How often you receive an Ability Score Improvement is dependent on your class, but is typically once every four levels.
Note that this increases your ability score, rather than your ability modifier. So increasing a score from 15 to 16 will increase your modifier from +2 to +3. But increasing from 16 to 17 will still leave you with a +3 modifier.
Other numbers (such as armor class) can also be improved, but you usually do so by buying better gear.
Yes, modifiers do typically scale with level.
This is chiefly accomplished through the idea of a proficiency bonus. (PHB p. 12) This starts at +2 at 1st level, and rises to a max of +6 at level 17. (p. 15) Obviously, this is a much more limited range than you'd see in previous editions of the game. (This is a concept the designers have called bounded accuracy.)
You add the proficiency bonus on the following, but only when you're proficient with them:
- Attack rolls with weapons
- Ability checks using skills and tools
- Saving throws
You also add it to attacks and save DCs related to spells you cast.