Briefly explain why it is exactly common to install two hard drives?

There's one M.2 and then one more on it. The M.2 can have around 500MB or more & the second in turn e.g. 1-4TB. What is the point of this M.2 & what is the point of the second? Everything I know about it, I know from the Internet, and for the last three days I have been busy with everything to build an expensive gaming PC myself. I will probably ask someone for help, for example, so as not to bend or destroy the legs of the processor.

Could you please recommend two SSDs to me, would be more suitable for SSDs than for a SSD & a HDD, since I have read that these are faster. Besides, you can add several more, is this necessary? I won't need that much data, I haven't had a PC for about 5 years & I just miss it. I really enjoyed playing on it back then, but now I'm out of the teenage age and want to breastfeed my gaming heart again. In the last few years I had a laptop that I played on from time to time, but it slowed down every week because it really isn't a gaming laptop, it was only for school, the usual MS Office programs now it is almost broken. I want to assemble one now just as a hobby and I would like it to last for the next ten years. I also make some music on the side and in the future I will also produce music on it, make beats, maybe cut videos, etc. But first and foremost I would like to play and listen to music.

I wanted to install the following components:

RTX 2080 TI Gaming X Trio
Ryzen 9 3900X
MSI MEG X570 Ace
Be Quiet Dark Rock Pro 4
G.SKILL 32GB, DDR4-3200
ASUS Essence STX II 7.1
Corsair Crystal 460X RGB
Be Quiet Dark Power Pro 11 (850W) or more / less?

Which parts are still missing? As already mentioned, I clicked through pages for days and gathered so much information that I believe that these components all fit together. I also paid attention to things like overheating and graphics, as I will soon have two weeks vacation and want to gamble continuously.

Thank you in advance for your help & would like to know what you think of the constellation and please don't forget my questions! Peace!

More memory

You basically only need one record.
It should of course be quick.
Fast AND big is still very expensive.

That's why many build a small (e.g. 500GB) SSD and for a lot of space, a large (e.g. 8TB) HDD.

Quite simply, a fast (SSD) disk for the system and the software, which you can quickly format and replace if necessary, as well as a second, larger disk for photos, music, texts, data that you do not want to lose you leave it untouched when you set up the system again.

Of course, you can also take a single huge disk and then divide it into several partitions, but then in the event of a defect all data is gone and large SSDs are very, very expensive.

It is best to get a large backup disk or cloud storage in addition to the two hard disks, where you can make regular backups of the data disk.

I have 2x the 970 Evo and 1x the 860 Evo, both from Samsung.

Why multiple SSDs?
Because separate and easier to use.
If I want to reinstall Windows, I only have to pay attention to the first 970 Evo because Windows is on it. And the separation between the 970 Evo and 860 Evo because the former is faster and more expensive and the latter cheaper but slower.
I also have an HDD, but it still flies out because I don't need it and it is also the loudest part in my PC,

I can recommend both models to others.

What would be the disadvantage or advantage of an 860 evo & only one 1TB 970 evo? Do I have to make sure that one of the two plates says m.2?

M.2 is just the form factor for the SSD, nothing more.
NVMe is the protocol that runs over PCIe and is significantly faster.
SATA is the older protocol that HDDs also use and is significantly slower.
NVMe is only available in the M.2 form factor, SATA is available in both form factors.

How many NVMe SSDs you install now does not matter and is up to you.

I have the separation because I save the reinstallation of games and co. When reinstalling Windows.