Please explain viewing of stereo camera ouptut

I ran into a local fellow with a few(!) stereo cameras that were in the family, so I could try to get one, perhaps.
I have no idea how to choose one over another either... I think most are 35 mm- one of them is a large format, which would be an adventure, a wee it above my pay grade!

What the viewing options are at this point, existing, or in need of being recreated?

The usual method was a slide viewer you held to your eyes. They did make stereo projectors.

See http://www.3dstereo.com/viewmaster/pj-tdc116.html

You can use things like this
https://www.amazon.com/Stereo-Wide-...ocphy=1013581&hvtargid=pla-646153989793&psc=1

or things like this, if you can find one
https://www.viewmaster.co.uk/htm/brumberger.asp

First, make sure they are in good working order. The Graflex and Kodak stereo cameras I've seen seemed a bit light and fragile. The Illoca's and Realists are fairly robust. I've not had the opportunity to look at other manufacturer's cameras. The Realist "format" seems to be the standard, but don't let that get in the way - you can always find a way to view the images. Do a little research to see what you may prefer. I plan to get a medium format stereo camera at some point, and one of those "old fashioned" stereo card viewers that you can still buy new kits for.

Being used to an SLR, I did have issues not getting pictures of my fingers, lol.

At the moment, I have a Realist 45 (made by Iloca) and a couple viewers for Realist slides, as well as the masks you put the slides into, though those are hard to come by and expensive, but they are still out there. I supposed you can make your own from cardboard, and there may be places making new ones from time to time.
Example of viewer:
http://drt3d.blogspot.com/2015/10/kodalide-ii-vs-realist-red-button-viewer.html

Example of masks:
http://www.3dstereo.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=MT-R21-22

I also have a Viewmaster camera, and you can find reels for those film "chips" on ebay sometimes.

There are times I've scanned the stereo pairs (film), positioned them in The GIMP to make a stereo cards, and uploaded to a printing company to have them printed on RA-4 paper. You can set the pairs up for "parallel" viewing and use a stereoscope:
https://en.wikipedia.org/wiki/Stereoscope
or "cross-eyed" viewing like those "Magic Eye" 3D books. I had done this with negative film when I first got my Realist 45 - here are the test shots, made for cross-eyed viewing (which I had printed on RA-4 paper):
https://photos.google.com/share/AF1...&key=Y0h6bzJ1SExXai12VGxXRFZYcmQ4M2gzcmd0ZzNn

I did the same when testing my Viewmaster:
https://photos.google.com/share/AF1...?key=a1g0c3B2UUt1UUJ6SllfemdWTDNlMEY2R0VsXy1R

Excuse my poor scans and composition.

You can also get a projectors as already mentioned, or do even more advanced things.

You can still get inexpensive plastic stereo slide viewers for Realist format new, on ebay or Amazon, etc. Note that the viewers for stereo slides and for stereo cards are somewhat different.

If you want to make stereo slides like a Realist format, then you need to shoot color or b&w reversal film, or have some way of making positives. (Or view prints, like on a stereo card). The need for transparencies was no big deal when shooting color slides was very common, but since most film use now seems to be b&w or color negative, is worth mentioning.

It depends....

Mostly though the stereo-camera is one thing and the kind of presentation another. Thus you get a (stereoscopic) pair of images and then can chose on which optical principle to present them.