There is a startup check that prevents users from booting any version of macOS older than what was current at the time the machine was first released. As it so happens, just yesterday I wrote up a long answer on how to bypass this check: https://apple.stackexchange.com/a/402727/4027.
I encourage you to give it a read, but in short, you need to create a bootable installer, and add your computer's model and board ID to a file called "PlatformSupport.plist". This will allow your computer to try to boot the old OS, and either succeed or fail on its own merits.
So the question is, will Mojave actually function on your MacBook Pro?
The 2020 13" Macbook Pros come with two types of processors. The lower-end models with fewer thunderbolt ports have 8th-gen Intel Processors, whereas the higher-end models with more ports have 10th-gen Intel Processors.
Mojave does run on other Macs with 8th-gen processors, so I think there's a good chance that this first group of MacBook Pros could run Mojave. On paper, the hardware is compatible, and someone ought to try it!
Unfortunately, from what I can tell, I believe you have one of the higher-end models with a 10th-gen processor. I wouldn't expect that to work unless you can find or make a custom kernel—and even then I'd expect other hardware issues, similar to what I encountered.