I am deciding to buy a new router to serve as the main router with OPENWRT and I want to enable a system such as QoS to control the bandwidth and I am looking for a router that is robust so that this without problems
TP-Link Archer C5 AC1200 v2
Xiaomi Mi Router 4A Gbit
netgear AC1750 R6700V3
netgear AC1900 R7000
netgear AC3000 R7900P
netgear AD7200 R9000
Suggestions?
How did you come up with the list above?!
Apart from one -maybe one-and-a-half- devices, it's a perfect example of what not to buy under any circumstances, if OpenWrt support is desired.
Your list of requirements also misses a rather essential detail, the WAN speed you're hoping to achieve.
Ok Is that they are the best equipment that I saw that had openwrt compatibility (they appear in the opewrt table https://openwrt.org/toh/start?dataflt[Brand*~]=)
I thought all these were fine because I looked for them in the table and they came out and they had good ram and processor and space ... I swear I thought they were good equipment 
Could you tell me which of those is good or another router that if it were good so it's not on that list I made? And my speed 20 Mbps down and 7mbps up if that information is useful, By the way, in the country that I live, you can find mostly tp-link... the netgears belong to someone who brought them on a trip to another country, but it is not common here at all.
You do need to look beyond that, into the individual device pages (hint, "unsupported functions").
c5v2, R6700V3 and R7000 are all Broadcom based, with unsupported wireless functionality (and that's not going to be supported in the future either).
R7900P is also Broadcom, but at least using fullmac wireless cards, which are supported - but which I wouldn't choose voluntarily in 2021 (yes, it works with OpenWrt, but brcmfmac and its corresponding firmware are rather limited, so advanced features such as WDS/ 4addr and similar interface combination don't).
The R9000 is based on the pretty exotic Annapurna Labs Alpine AL-514 SOC, yes it's fast and has good WLAN, but it's totally unsupported mainline or in OpenWrt at this point (while there has been some renewed activity at Netgear X10 (R9000) recently, I haven't seen any push to submit it to mainline/ OpenWrt either - so at this point, its future would be bleak at best).
From that list, only the mir4a with its mt7621a SOC and mt7603en+mt7612en would be fully supported by OpenWrt as-is, but it comes with its own pitfalls as well (e.g. rather difficult installation method, limited features, 2+1 1 GBit/s ethernet ports, no USB - and easily mistaken with the 100 MBit/s variant of (almost) the same name).
The problem with your list really is that you perfectly nailed all the devices that don't work properly, most of them Broadcom based (and quite a few of them rather old/ EOL) - as if you made a specific effort to collect the worst of the worst. Any of the vendors you listed above do produce good devices, but with OpenWrt in mind, you need to pick wisely among their portfolio (read the full device page for the devices in your selection first) to get fully supported models.
For 20/7 MBit/s, pretty much anything based on the contemporary(!) ath79, lantiq vr9, mt7621a, ipq40xx, ipq806x, mt7622 SOCs will do.
soorry sorry I really thought I was choosing them well...
I will try to find others and enter the individual page to review the unsupported functions
Btw., if you are aware of the limitations (number of ethernet ports, no USB) of the mir4a and can cope with its slightly convoluted installation process, this is the only device from your list that would fully work and meet modern expectations (but there probably are better/ easier alternatives as well).
ill try to look for another routers