The Synology's are nice - my cousin has this setup for his NAS.
Personally, I put mine together using some old PC parts and I populated it with a bunch of HDDs. I installed TrueNAS Scale which is a freeware, enterprise grade NAS OS. I'm very happy with it, but I think it's overkill for what I need.
I also put together another NAS using an old Netgear ReadyNAS. This was just to play around with, and it is too underpowered for what I need it. But for this test system I used Unraid. The OS is a bit hard to navigate, but once you figure it out, I think I like Unraid better than TrueNAS.
My NAS was originally designed to offload my photos, videos, audio recordings and data. But I got hooked on the *ARR self-hosting applications, and now I am using it as a customized content-streaming. Another nice feature is hosting an image-sharing cloud storage through Immich. This allows me and my family to backup their photos from their phones, and delete them from their phone storage. This obviates the need for iCloud, Google Photos, etc.