Player Characters gain experience by finding treasure and spending it in town. Every 1sp spent in a town, city, or other (mostly) safe place earns a PC 1xp.
Only money earned during adventures counts. In specific, money earned in risky situations where death is a real concern. Money earned in other ways (say investments or a trading caravan) actually subtracts 1xp for every 1sp spent by a character. This will never reduce a PC’s levels, but will make it harder to gain the next level. This does not count for money earned on a trading route through dangerous territory that the Player Character personally travels with and protects. It’s all about taking personal risks and playing it safe.
Something to keep in mind is that a lot of treasure is art or items of historical value. These can get a decent return just finding a merchant that will buy them but many can be sold for much more to the right collector. So keeping a piece of treasure to find just the right person to sell it to is often a good idea.
Clerics can donate to the local church. Fighters can buy training from a local weapon master. A fallen noble might send money to rebuild his family’s estate. Wizards and Shapers can build laboratories. There are near endless ways to sink money into things for exp purposes. All of which will have some positive effect.
If one wishes to boost the experienced gained, a PC can spend a haven turn carousing to get a multiplier on the experience they gain from the money spent. Though this might get the PCs into some kind of excitement from partying too hard.
Writing adventure logs is another way to gain more experience. Each in-character adventure log, a journal entry, or entry that is written as a story told at a tavern later, or other such in-world format, will boost the experience gained next time the PC is in town by a small amount.
Every class uses this chart for the experience points needed to gain a level. The requirements below are total, so a character at 4th level that wants to get to 5th level needs to earn 8,000 experience points on top of the 7,000 they already have.
All classes use the same chart, but PCs will often have different rates of exp gain even with same amount of sessions and/or treasure because of decisions made. Carousing, keeping art objects to sell to collectors, etc.