DIMMs that are labelled Samsung, SK-Hynix and Micron are the three I usually go for, or the major system manufacturers like Dell, Lenovo, HP that re-badge those three makes as their own, and Kingston. I expect Crucial are fine but I haven't used them. I had some Elpida (japanese) ram years ago that was fine too, but I think Elpida went out of business. I wouldn't touch anything else. If the motherboard manual says it will work with PC4-1700P-R or whatever spec they state, it should work fine with dimm's of that spec from any of those three main makes. The 'approved' dimms they list for the motherboard, are the ones they have actually qualified, ie the ones they have tested it with, but you should have no problem with any of the big 3 (samsung, hynix and micron) provided they are of the same spec.
For example, below is a classic samsung DDR4 32GB ECC RDIMM. The main spec you can read on the label is PC4-2400T-R, meaning DDR4, the bus speed, and registered (homework: look up the difference between '2400' and '2400T'). You can tell it's ECC because there are 9 sets of chips instead of 8 that you would find on a non-ECC (ie, desktop) DIMM (at least, that rule of thumb works for DDR4. DDR5 is a bit more complicated), and you know it's registered because of the extra chip in the centre. Note that in this case we have a Samsung DIMM and the DRAM chips you can see are also from Samsung, labelled 'SEC' which is Samsung Electronics Corp'. There is a useful guide here
https://rampricesusa.com/guides/server-ram-guide . If you're lucky and it has a FRU number on the label you can search for the FRU number to check the exact specification. The wikipedia page for DDR4
https://en.wikipedia.org/wiki/DDR4_SDRAM describes the meanings of the JEDEC standard terminology (see the table under 'JEDEC standard DDR4 module'). Just make sure you get the same JEDEC spec as the motherboard web page says it supports and you should be fine. Also check if you need any special type like low-profile dimms to fit the case. The other rule is generally to put all the same type on a given motherboard, so if there are 8 slots then fill them all with the same type dimm, don't mix different types. So if you have some slots already installed, just buy some more of the same spec. that you already have. If the mobo data says it supports 3 different specs, for example, then choose one spec and populate it with dimms all of the same spec. That's what I do, anyway
(Answer to homework:
https://superuser.com/questions/1291897/whats-the-difference-between-pc2400t-and-pc2400-memory . It's just a more precise timing spec.)
but I couldn't run 4x 3666Hz rated sticks because of that and/or 2700X/Zen2)
