Here's a quick breakdown of the naming conventions from lowest to highest:

NVIDEA cards (most recent series):

RTX 5050 - 8 GB vram - 💩

RTX 5060 - 8 GB vram - tight budget / entry level card

RTX 5060ti - 16 GB vram - mid tier 1080p/can do 1440p resolution gaming at lower settings

RTX 5070 - 12 GB vram - 1440p on a reasonable budget

RTX 5070ti - 16 GB vram - top of the midrange with a very comfortable amount of vram

RTX 5080 - 20 GB vram - high end card (4k gaming decent fps at high settings)

RTX 5090 - 32 GB vram - enthusiast card (overkill, if you don't know why you'd need it you probably don't, also good luck finding one at a reasonable price)

(VRAM is how much memory the card has, higher resolutions require more vram, but excess vram won't make your card faster. It's a threshhold, either you have enough or you don't (higher quality settings or a higher resolution = need more vram. More vram MIGHT mean that if in the future you want to play games requiring more vram you'll have it on hand (i wouldn't worry about that though))

AMD Cards (most recent series):

AMD Radeon RX 9070 XT - 16 GB vram - high end card (4k entry point, at high settings might be able to pull 60fps)

AMD Radeon RX 9070 - 16 GB vram - high/upper midrange card

AMD Radeon RX 9060 XT - 16 GB Vram - budget/midrange card, decent (comparable performance than a 5060ti 8gb version but with more vram) high settings 1080p resolution, can do 1440p.

I would recommend getting the cheapest AMD Radeon RX 9070 XT you can find (don't worry too much about the aftermarket brand, (companies buy gpu's from nvidea then "trick them out" and put fancy fans on them, it's the same gpu with different cooling features on them). It will be inside your budget of $500-800. You should be able to find one around $750 ish. The reason is it is comparable to the NVIDEA 5070ti in performance, and has the same amount of VRAM, but is about 250 dollars cheaper.

Now, the difference between NVIDEA and AMD for someone like you (novice at computer building), is that NVIDEA is like buying an iPhone, and AMD is like buying an Android phone. There are some bells and whistles that the android might not have but you get more bang for your buck. An iPhone you'll end up paying a hefty premium for equivalent performance, or end up spending with an inferior model compared to the competition while having the same budget.

To be more specific NVIDEA cards are better at raytracing (light reflections), but I suspect that that won't matter as much to you as someone who's just getting into building their first desktop. I think you would be happier with the greater performance per dollar you'll get out of an AMD card, it will give your build more longevity than if you stepped down to a NVIDEA card with less power to stay within your budget.

NOTE: this recommendation for a gpu is solely based on your budget and not the other components you've selected. I have not reviewed your posted build. BUT regardless of your other components (assuming your case is atleast big enough to hold the gpu) I would still recommend the 9070 XT regardless, because if you ever want to upgrade other components in your desktop you can always move the gpu into the new build, or over to a different desktop.