Rolleimat Universal enlarger aperature help?

I was given a Rolleimat Universal enlarger and am trying to figure out what it is missing... I am new to film photography and have successfully printed several images using this enlarger. But as I've been reading more I learned that you can change the aperture on enlargers? This one doesn't seem to have any way to change f stop. Do I need to change the lens? Where is the lens carrier...? I have a lens I was trying to attach but see no way to do so... I'm also not sure what some of these parts are for (the black cylinder with orange plastic, there is a piece of a lens... not sure where that is from) I'm also not sure if the filter holder and red filter are even compatible with this enlarger. I can find no way to attach...

Can anyone help me see what am I missing? It must be something simple. I tried to find a manual, any forum posts or photographs to see the set up but can't find anything helpful. this is the only enlarger I've used so I don't have the experience to know what I'm doing! Thank you in advance

Strictly speaking the typical enlarger has no aperture setting, the enlarging lens has. And, enlargers with dedicated lenses aside, the enlarging lens is seen as unit on its own. Anyway, a enlarging lens has an aperture and a means to set it. At the barrel of the lens. In your photo I see parts of an enlarging lens. One part seemingly still mounted in the enlarger. At a part located on the base board one can see white inscriptions. Here the aperture ring is located to set the aperture.

HI AgX,
Thank you for the response. I know I have an enlarging lens (separate) but I can't figure out how to attach it especially since there already seems to be a lens in the enlarger? Any tips for how to remove the existing lens to replace it?

Enlargers differ in the way they take the lens: some got a respective lens thread, others, especially those for larger film formats, got proprietary, exchangable holders with different lens threads respectively. You have to sort this out. Those holders are designed for easy exchanging. I once had a Rolleimat colour head in house to salvage for parts and to my surprise at least the motorized focusing was of flimsy design.