Rounds give a fixed amount of XP in BTD6, and that XP is distributed based on how much money you have invested into all towers. Pops do not factor in for experience gain or distribution.
Tower XP is granted based on rounds, using the formula found here:
https://www.bloonswiki.com/Tower_XP20
r+20 for
r ≤ 20
40(
r−20)+420 for 21≤
r ≤50
90(
r−50)+1620 for
r >50
where
r = Round
XP is distributed based on amount of money you have invested in all placed towers, excluding your hero. For example, if you have invested $1000 total into all towers, $400 on dart monkeys and $600 on wizard monkeys, the XP will be split 40% to the dart and 60% to the wizard. Choosing a harder map will multiply the XP gained: x1.1 for Intermediate, x1.2 on Advanced and x1.3 on Expert.
This is fairly easy but long grind once you identify a monkey you wish to use as your main. Once you are at a point where you can easily pop bloons, begin buying multiple monkeys of your chosen level up and buy their most expensive
reasonable path. For example, I used the wizard monkey and once I got a 0/2/4 and was using the necromancer ability to easily pop bloons, I would begin buying 2/4/0 wizards since that is the most expensive upgrade path.
Some guides recommend running deflation on an expert map as a set it and forget it grind, but I can't say I used that method so ymmv.