Growth

When a character gains a level, they roll dice equivalent to their hit dice and gain a permanent HP increase equal to the result.
During each level up, a character must also choose to increase 1 level in a class they have access to, and 1 level in a subclass (if) that is available to them. Characters can only gain levels in 1 subclass per class.
The character also usually gains all features, abilities, or traits associated with that class and subclass level upon reaching that level, however there may be some instances in which the character may need to achieve additional objectives to unlock new abilities or features.
When a character reaches level 4, 8 ,12, 16 and 20 they may choose a feat or ASI to incorporate into their character. Class levels no longer provide feats or ASI at lvl 4, 8 ,12, 16 and 20. However feats/ASI tied to specific classes such as Rogue (lvl 10) and Fighter (lvl 6 and 10) still get their bonus feats/ASI.