100G on singlemode (100G-LR4 being the most common) uses the well-known two-strand ("duplex") fiber. Or you can get 100G bi-directional ("BiDi") over a single strand of singlemode (fiber-to-the-home often uses this).
100G on multimode is weird. As the name implies, one beam of light, sent down the core of a multimode fiber, results in multiple modes (search "Laser modes") being sent down the strand. As they overlap, it gets hard to get a clean signal out the other end.
To deal with this issue, 100G on multimode uses fiber cables containing multiple strands per direction of travel. MPO-8 and MPO-12 are common cables used for 100G multimode: It contains eight or twelve strands of fiber. Four strands are used to send, four to receive. And the prices for those cables are higher than standard duplex singlemode cable.
OS2 transceivers are not that expensive anymore with the rise in third-party modules: sure, first-party, OEM SKUs are pricey, but a home user is not going to go that route.
Standard /r/networking advice is to go single-mode basically everywhere due to price drops over the last 10+ years.
The math worked out just to move to single mode a decade ago
The sibling posts have already pointed out the pricing has mostly equalized at this point too, especially if you're more realistic and look at 10G or 25G. (Or just buy a pile of used 100G CWDM4 transceivers off eBay.)
