Games : How does EXP work?

I'm playing Pokemon Shuffle and I need to grind my team a little more for harder levels.

So I'm now wondering about the leveling mechanics. They are quite vague, you can't tell how much experience a Pokemon gained during a match, if there are certain factors to how much they gain...

For example:

- Do my Pokemon receive EXP if they're just in my party or do they only get it from combos?

- Should I focus on higher levels or do they level up the same for no matter which level I choose?

- Are Expert Stages a good way to grind?

The only thing I know is that the Pokemon who does most damage in a match gets a crown icon with boosted EXP. But I don't know how much.