Does it have to be the exact same ram?

I have a ram with 8gb and 2133mhz in my laptop, if I want to buy 8gb ram for the second slot, must it be the same (same brand, same Mhz)?

or is it enough if it has any 8GB ram with at least 2133mhz.

You can also add 4GB 2133MHz. In the end it doesn't matter, everything just runs at the lowest speed of a stick. If you add a 3000MHz stick to it, it will not bring you more or less.

With 8GB you would be in DualChannel and that would be the best option

But I don't have to pay attention to the brand, just the Mhz and GB right?

The exact same thing would be absolutely safest. Nothing can go wrong.
In principle, any DDR4 SODIMM works. A 16 GB 3200 MHz would work just like a 4GB 1600 MHz. As long as it is DDR4! And the chipset will take the lowest common denominator. So with the 1600 both would run on 1600 and with the 3200 both on 2133 :)

Strictly speaking only on "DDR4 SO-DIMM"

Correct

I would say it doesn't matter which brand the RAM comes from. Only where you have to pay attention are the units. If all units agree one to one, that's fine.

If you use RAM with different performance, the BIOS in Windows and the boot loader in Linux focus on the poorer unit. This is because otherwise the other RAM modules could break.

Yes, there are 2 different types of RAM modules.

You can populate RAMs with JEDEC as you like. But if you have a RAM bar, which values are better. Eg 2133mhz and 2400mhz, it is clocked down to 2133mhz. This is also the case with the CL value.

RAM with XMP must be exactly the same with the other RAM.

Of course it has to be the same RAM stick! For example, in a car Also do not attach a large tire on the left and a very small tire on the right…