Their competition is the enterprise hardware divisions of HP, Dell, and IBM. SuperMicro makes reasonably good quality, lower-cost server equipment. They are, IMO, a pretty good value if you don't want high-end support from the hardware vendor.
There's a bunch of other kit too, but https://servethehome.com reviews a bunch of the various rack systems.
Example of some late January posts. Albeit none are of the "fits lots of GPU" sort that is helping propel Supermicro, but these folks all have those offerings too,
Supermicro SYS-511R-M Intel Xeon E-2488 1U Server Review https://www.servethehome.com/supermicro-sys-511r-m-intel-xeo...
Gigabyte R183-Z95 Review Dual AMD EPYC Server with a EDSFF Twist https://www.servethehome.com/gigabyte-r183-z95-review-dual-a...
ASRock Rack ALTRAD8UD-1L2T Review This is the Ampere Arm Motherboard You... https://www.servethehome.com/asrock-rack-altrad8ud-1l2t-revi...