This question gets messy fast.
You are confusing the frequencies and the names of the wifi. Maybe better to call it 802.11ax rather than wifi6.
So there are in general 2 radio bands 2.4 and 5ghz and a new one that runs on 6ghz. Note wifi6 does not use the 6ghz radio band that is wifi6'e'
This has nothing directly to do with the type of data encoding it is running on those radio channels.
Depending on how old your router is it can be running say 802.11n, 802.11ac or maybe 802.11ax. These would be wifi4,wifi5,wifi6 but best to use the more technical names.
What will happen is if your router only supports 802.11ac and your pc supports 802.11ax. Since 802.11ax is backward compatible it will drop to 802.11n. So in...