Linux

my knowledge of technical stuff is bretty basic so please be patient with me.

First of all, just from the fact that you're posting here and asking that kind of question, your knowledge of technical stuff is at least a little bit beyond "pretty basic."

Second, I get the impression that confusion over exactly what you're asking for is maybe more due to English not being your first language...? (No judgement if that's the case; your English is certainly way better than I could do in a second language.)

Anyway, on to actually giving my answer to your question:

• Trying to set up a multiseat system can be tricky in general. If I recall, Other Linus (the one from Linus Tech Tips) has released several videos about that sort of thing over the years, but I don't think any of the tries were successful enough for him to daily-drive long-term. I know LTT is controversial, but it might be worth taking a look at his experience.

• Trying to do it with GPU passthrough for gaming and 3D modeling adds an additional layer of complexity. I'm a software engineer and have been using Linux exclusively at home for almost a decade (and off and on for many years before that), and even I don't have GPU passthrough working on my home server. That's not necessarily to say that it's super difficult -- I haven't tried very hard to figure it out -- just that it isn't trivial even for somebody with experience.

• If the above has scared you off from the whole "multiseat home server" thing but you still want a home Linux PC for gaming, my distro recommendation would be either bazzite, which I haven't used, but have heard good things about its appropriateness for that use-case, or boring ol' Ubuntu (or variant like Kubuntu, depending on your UI preference), which is popular enough to have official support from corporations like Valve and AMD and thus is most likely to "just work."

• If the above didn't scare you off from building a home server, I recommend running Proxmox on it.

• As for Nvidia, I fucking hate Nvidia for its CUDA monopoly and would never recommend it out of principle, but I have to grudgingly admit that some stuff just flat-out won't run on AMD or Intel GPUs. I believe proprietary "niche 3D software" is one of the most likely things to fall into that category, so you may have literally no choice. Check the system requirements of the particular software you plan to use.

• The other features that you might lose out on by not using Nvidia are raytracing and hardware-accelerated PhysX. The AMD 9070 XT allegedly has decent raytracing, but although I own one I haven't verified that yet because I don't own any raytraced games. I tried the Half-Life 2 RTX demo, but it failed to start at all. As for PhysX, there are two important things to know: first, that should be improving because Nvidia is working on open-sourcing it. Second, for older games using the older PhysX API, the new 50-series Nvidia cards don't support them either. Apparently, if you want a decade+ old game like Mirror's Edge to work properly on your 5090, you've got to also have some cheap older Nvidia card alongside it to offload the PhysX calculations to, LOL.

• Speaking of multiple cards, if you want to build a server that supports multiple GPU-accelerated users at the same time, you might consider getting multiple cheaper GPUs instead of one 5090. Although I believe virtually slicing a single GPU for passthrough access by multiple VMs at once may be possible in theory, the phrase "may be possible in theory" should be setting off alarm bells in your mind that it ain't gonna be easy.