I'm not the best person to explain this, but I'm going to try. I'll link other explanations if you dont get me.
The BIOS revision (sometimes referred to as UEFI firmware revision) is some code flashed onto a chip on the motherboard. This code determines a lot of things including what CPUs are supported by that particular motherboard.
When B360 motherboards were first released, 9th generation CPUs did not exist. Because of this, the bios revision on the motherboard did not have support for any 9xxx processors. If you tried to use a 9xx CPU in a motherboard with a bios revision without 9th generation support, the motherboard would not detect a supported cpu and would not boot.
If you buy a B360 motherboard now, it should have a newer BIOS revision that will support 9xxx CPUs, so a 9th gen processor will boot no problem. The issue is that if a B360 motherboard has been sitting on a shelf somewhere for a while, it may have an older BIOS revision and will not boot with a 9xxx CPU. This is a common confusion.
I suggest asking the seller of the motherboard if the BIOS revision is updated for 9th gen support. If it is, your ready to rock.
If not, you will have to buy a 8xxx cpu to proceed with the tedius process of updating the bios revison. If this process fails, the motherboard will be bricked and nonfunctional.
So my suggestion is to do the following.
1) Confirm the seller says the B360 motherboard has an I7 9700 or I5 9400 compatable BIOS.
2) If you want to be extra safe, you can purchase a newer but slightly more expensive B365 motherboard which is 9th generation compatable regardless of BIOS revision. (the 5 at the end is the difference)