It looks like all your plumbing is already tied into the pit, the pipes that you see in the wall this is the vents pipes.

A floor drain will have a trap in the floor under the concrete. A toilet drain won't (the trap is in the toilet).

As Greg suggests, verify that the drain in the middle of the floor has a p-trap. Otherwise, bust out some concrete and put a trap in. The 4" pipe closer and on the left looks to be the right rough-in distance for a toilet (so be sure to purchase the toilet accordingly). Make sure that pipe doesn't have a trap in the floor. Connect your vanity drain to one of the vertical vent pipes using a sanitary tee. Check the mfr labeling on the vent pipes, I suspect you will find they are 1-1/2" lines.

Is it to code to tie my vanity sink drain into one of the vent pipes?

No, you are correct, it is not code to drain into a vent pipe. If you call the existing piping 'vent' pipes, what is currently being vented? Nothing. No plumbing is installed as yet. What you have is rough-in piping, nothing is finalized, it was all pre-done to make a bathroom install at a later time possible. And, it was all connected together and closed so there would be no issue with possible sewer gases from the ejector pit (note that its vent is connected to the house vents already). Now that you are finishing up the plumbing, the moment you insert a sanitary tee, the line below the tee becomes a drain and above it is the vent line. Magic! Also, the 4" toilet drain is self-venting as is the 4" floor drain; they don't require a separate vent line.

Also, the 4" toilet drain is self-venting as is the 4" floor drain; they don't require a separate vent line.

I'm pretty sure this is incorrect. All drains need a vent. If the toilet is close enough to the main stack, then the vent on the main stack may suffice, but it's still vented.

I'm sure two of those pipes on the wall are venting the 4" drains and the third can probably be used for a drain. But which is which?

Otherwise, what would the purpose of installing those 3 pipes be? There'd only be one.

The very left one is probably the vent for the 4" drain that it's closest to. The other two might tie into the drain at a similar spot, so using either one will convert it to a drain and the other is the vent for the other 4" drain. So it may not matter which one of those you use.

But I'm not a pro. It might be a good idea to consult a plumber.

Check old plumbing posts from lazypup; he's gone now but he knew plumbing inside and out and taught it well. "All drains need a vent" is not a true statement, toilets are self-venting, they don't need a separate vent. A 4" floor drain would also be self-venting. Self-venting occurs because the inside pipe diameter is large enough that waste can flow downhill (it's all slower gravity flow, not high-volume pumping) with enough room above the liquid level for venting to take place. Self-venting is also applied to sink drains. For example, at 1/4" slope per foot, a 1-1/2" drain line has a maximum trap arm length (max length before venting is required) of 6 feet. For 2" drain lines this length becomes self-venting up to 8 feet. Regarding those 3 pipes you have in the wall, it is not against code to install extra drain/vent lines, which is what you probably have, but it is not needed.

Maybe it's just the terminology that we are getting hung up on. All drains need a vent but the distance that the drain can be from the vent is dependent on the drain size.

You call a 4" drain "self-venting", but I'd say that if it's within 10 feet of the main stack (which is the vent) then it's using the main stack as the vent. I have never heard that referred to as "self-venting", but I think we mean the same thing.

If it's 11' from the main stack, then it needs a closer separate vent.

So you're right, the closer 4" drain is certainly close enough to the ejector pit that it doesn't need an additional vent. But the further one may need one. I'm not sure what that distance is. And I'd guess that the vent closest to that pipe is required. Or at least a good idea. There's no such thing as too good of a venting system. :)

thank you for all the feedback. I’d like to also add that there is an existing floor drain in the utility area that my water softener and humidifier drain to.( drew it’s location with the red arrow). Could the third pipe on the wall closest to the ejector pit be the vent for that floor drain. that floor drain drains into my ejector pit aswell.

so am I good to tie into either of these for my bathroom sink drain or would one or the other be a better option or up to code.

pipe circled in blue or red?

also just checked the pipes coming out of the floor, the one to the left near wall that I’m planning on using for the toilet does not have a trap and the one to the right that I’m planning on using as a floor drain does have a trap so that is all good news.

I want to thank everyone for all your input. Just got my plumbing roughed in for the basement bathroom sink.

appreciate all the advise.