In my experience, based on a Quiet Pro, a lot quieter than some of those YouTube videos imply. It's not scissor switch quiet, but it's about as quiet as you can get without using scissor switches.
I'm not too hot on the feel (they're rather grainy/scratchy), but the Quiet Pro truly delivers on sound — it's extremely quiet. Part of that is down to how Matias have designed the stabilisers — you don't get the usual clang from those, either. Off-axis strikes can be fairly loud, though — probably means that you're overreaching the limits of the rubber dampers.
Alps imo is an old competitor to Buckling springs of its day. I guess other competitors might have also been Beam spring and Hall effects as well but maybe predominantly it was Alps versus Buckling spring.
IBM Selectric (electromechanical) was superseded by beam spring, which was superseded by capacitive buckling spring (IBM Model F keyboards), which was superseded by membrane buckling spring (IBM Model M keyboards). None of these rivalled each other — each one replaced the last.
There were probably hundreds of switch types in the 70s and 80s: Cherry M6/7/8/9/10/11/MX/MY, SMK linear/2nd gen, Futaba linear, too many Alps switch types to count (Alps went completely nuts), ITW/Cortron electromagnetic, Micro Switch/Honeywell Hall effect, IBM as mentioned above, Hi-Tek/Stackpole waffle frame/dovetail, Stackpole discrete (for which there are several patents!), those little MEI laggy band switches, NMB Hitek, PED, Omron B3G/B3G-S series, Mitsumi mechanical/mini-mechanical/hybrid, Apple's own
hairpin spring switch, amongst many other known switches and dozens that we don't even have an ID for yet. We still don't know what the ones in the Fluke Y1700 are — possibly something relating to the ITW/Cortron patent based on Hi-Tek dovetails, but still unproven.
The most popular types of the day are probably IBM membrane BS, SMK linear, Alps low-profile oval mount, Cherry MX and Alps SCKL/SKCM, in no particular order. Cherry MX was hugely popular upon introduction in 1983 and is one of the very few survivors.