If you are trying to earn BP points to unlock more skills and grow stronger, you will need to level up. Some ways to level up in the game are to progress in the story and complete side missions. Along with this, exploring areas you have already been through is another good way to rack up some extra EXP.
Doing all of these will earn you a decent amount of EXP, especially if you farm side missions whenever possible. However, by far, the best source of EXP is the enemies in the game, and the more you slaughter, the more EXP you will earn.
You can also do a few things to increase the amount of EXP you earn from killing the many other enemies in the game. The first is to unlock a skill on the brain map called overkill.
Overkill is the second support skill in the game, and once unlocked, it increases the amount EXP earned when hitting enemies stunned by a depleted crush gauge. There is also a skill that you can unlock in the same tree as overkill that will give you more EXP when you brain crush others.
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Along with the skills, building up your brain drive bar by either killing enemies or taking damage. After the bar fills up, it will activate on its own, and while you are in brain drive mode, every enemy you kill will reward you with some bonus EXP.
You can combine this with a good farming location at Kunad Highway Junction to farm a ton of EXP by using the buses to instakill others with ease.
That said, start by finding Kunad Highway Junction and finding the spot shown in the image below.
Once at Kunad Highway Junction, you will be able to use the various buses spread about the area to kill others instantly, thus earning EXP with practically no effort.
When you slay the enemies, you will then need to leave the entire area, as enemies will not spawn otherwise. After this, you can farm EXP by returning to the spot in Kunad Highway Junction and farming the others in the same spot, rinse and repeat.
For more on Scarlet Nexus, check out Where to find Auger Sabbat in Scarlet Nexus? and How to give gifts in Scarlet Nexus?