I've read a lot of the archives on this topic but now I am more confused. Some say use it when it is as fresh as possible.
Yes, also if making yourself refrigerate as soon a pH hits 4.7.
Others say it doesn't have enough bacteria straight from the store.
Who says that? it's wrong. From the store, you are a race against time to get the freshest (latest expiration date) cultured buttermilk possible.
Let sit 8-12 hrs at room temp first, then freeze.
Well, if you're using the buttermilk as a mother culture and are making your own buttermilk, then yes, you need to let that fresh batch do its thing, and then freeze. But not the store stuff, you use that as is.
It is is sitting open in the fridge will the bacteria multiply (will it ripen) just at a slower pace?
In the fridge, the bacteria are slowly dying. No, there is zero multiplication, maybe a few thousand stray bacteria still have enough umph, but generally there's no multiplying in the fridge.
If so, isn't a bit older better?
No, use the freshest you can.
Also, I saw a recipe (with pictures! Thanks!) of making your own starter adding buttermilk 1:3 or 1:4 with milk and letting it sit out. But the pic showed Kroger brand buttermilk. I bought that once. It had all sorts of gums, tapioca starch and I don't know what all. Does that not matter?
Not really, that's just to prevent separation of the whey in buttermilk. So long as it's cultured, you're good to go.
I've made an effort to get one without all that. But is that necessary?
Only if you have an ethical objection to the additives. They make little difference for a starter.