I am building a new computer with Asus ROG Crosshair VIII Dark Hero X570 and Ryzen 9 5900x but I don't know if I can put ram with 3600hmz. I see some people using 3600mhz but I don't know if they are using overclock and some people using 3200mhz and I don't understand what difference 3600hmz vs 3600hmz (O.C)
You should try to use parts on the Crosshair VIII Dark Hero Memory QVL, if possible. Make sure you select your CPU series at the top of the QVL, as it's different for each CPU series. The board has been out for a while now, and DDR4 is last generation, so you might not easily find everything on the QVL. As a substitute for the ASUS QVL, the good memory vendors (e.g. G.SKILL) have their own QVLs which give you recommendations for which of their DIMMs should work on specific boards.
All XMP/DOCP RAM is overclocked; it's an automatic overclocking feature, and the standard JEDEC speed that you get with XMP/DOCP disabled is the rated speed of the silicon on your DIMMs (which will likely be somewhere in the 2133-2666 MT/s range). For Ryzen 5000, 3600 MT/s shouldn't be too difficult, and is even possible as a 4 DIMM config. E.g. I'm running 4x16 DDR4-3600 in my Crosshair VIII Extreme with a 5950X, with Crucial Ballistix BL16G36C16U4B.M16FE1 DIMMs (Micron have sadly killed off their Crucial division, so those might be hard to find now). 3600 MT/s RAM lets the Ryzen Infinity Fabric and RAM run at 1:1 speed (1800 MHz), which has performance benefits. In theory, the Ryzen 5000 Infinity Fabric can go as high as 2000 MHz (so DDR4-4000 RAM), but that is pushing things and might not be possible on all CPUs or with larger memory configs. DDR4-3600 is a nice sweet spot for both Ryzen 3000 and 5000, and DDR4-3200 is the slightly more budget-friendly alternative.
If you're going to try to run a 4 DIMM config, it's better to buy a single kit of 4 DIMMs (if possible), as combining multiple kits can be problematic.