Yep. You’ll want a base that’s actually slightly below the ground level in order for the bricks to settle and have a permanence that’s much sturdier than just stacking the bricks around on top of the ground. This also helps to level the pit. Don’t worry too much about your digging being perfectly flat. The next step will help with that.
Pea gravel is perfect for being the next layer since you can smooth it out as needed until it’s perfectly level. You can actually use a few cardboard boxes that you’ve broken down and lay them on top of the pea gravel and push where needed to flatten out sections all at once if needed.
This is what I do next, though some people may not agree with. I like to take 2’ by 2’ pavers and place them in the shape of what I want my pit to look like, but this only applies to the outside edges of where the bricks will be sitting. This is just to give the actual pit bricks a solid base to sit on. Again, use those cardboard boxes again if the gravel shifts from the heavier pavers.
These are primarily for keeping heat in above the ground. I layer these between the pavers in a way that makes the base all touching. You’ll want these to be in the center of your pit for the main reason of keeping heat in and not seeping down. Also, these are fireproof and won’t scorch any parts underneath.