If I have only 1 x8gb DDR4 2666 MHz, and I want another 8 gb, do I have to buy any Ram thats DDR4 with 2666, but it does not matter from what manufacturer?
No, you caan not buy just any ram and expect that it will work.
If you want 16gb, buy a 2 x 8gb kit of whatever speed your motherboard supports.
Sell the old 8gb stick, or keep it as a backup.
Ram is sold in kits for a reason.
A motherboard must manage all the ram using the same specs of voltage, cas and speed.
The internal workings are designed for the capacity of the kit.
Ram from the same vendor and part number can be made up of differing manufacturing components over time.
Some motherboards, can be very sensitive to this.
If you just buy a second stick, it may work, Your odds are good.
But, if it does not work what is your plan B??