Buildapc

Any compatible motherboard generally works for the CPU.

With AMD, this is basically a non-issue but high-end Intel CPUs are so incredibly power hungry that a motherboard VRMs can become a limiting factor. More money isn't always better here though; a 120€ board could be better than a 300€ one. You'd have to look up the specific board.

Most important though is feature support which mostly boils down to what I/O you need. E.g. NVMe slots, expansion cards, thunderbolt, networking or even just how many USB-A ports there are.

I don't have any specific requirements here, so I've so far gone with one of the least expensive boards that isn't utter trash and I've had no issues.

but I checked the CPU benchmarks of other AMD processors at that price range and also more cores.

Which benchmarks? There's a notorious site out there that has "benchmarks" so biased to the point of being as good as non-factual.

Hardware benchmarks are not a simple topic, so any one number that you see presented as "the truth" will be wrong for a thousand reasons. Please always use real-world benchmarks that closely resemble your actual projected usage (i.e. the games your friend likes to play) for gauging hardware performance.

My friend wants the PC for 3D rendering and VR stuff, so more cores seemed better in my eyes.

That's good to know. VR doesn't need any more CPU perf than regular gaming but 3D rendering can. It highly depends on what kind of 3D rendering your friend is doing though as you'd typically do that on the GPU; preferring GPU power even more than games.

Which specific software is this? Some software can't do GPU rendering but i.e. Blender can (and you certainly want an Nvidia GPU for that). You'd also probably want more VRAM then.

Also, are they doing this as an actual hobby; spending significant time on it or is it just a side interest? The latter use-case can be satisfied by any reasonably powerful system, the former justifies more investment.

In case you should need to reinstall the OS for whatever reason, I have more peace of mind knowing that data is on a different drive entirely. It can also save time if you're upgrading your system you don't have to download games again, for example, though you could just bring an old drive into a newer system and likely achieve the same, but to me it's simpler to have it separated, and then you don't have to worry about booting from the right drive. Which probably answers your last question, no I do not dual boot but if I did I would probably have at most 1 drive for data and 1 drive per OS (though most likely I would just partition a single drive for OSs).

I didn't ask who had the monitor or where it was, we need to know the model/specs to determine the appropriateness of the computer/card that will driving it. If you're doing all this to pair with a 15 year old 60hz 1080p monitor you're wasting everyone's time and your friend's money.

If your friend doesn't care about looks that case is a collosal waste, just get a black box with decent intake filters and nice fans for half the price.