The OP has a 2010, which will easily go back to the original Snow Leopard. But, as it is unlikely the OP has a bootable copy of a Snow Leopard that was not a general release version, then it would just be a simple matter of finding a copy of the installer for anything from Lion (10.7) to High Sierra (10.13). Make a USB bootable installer from that installer app, and go for it - install the system.
OP also said that the MBAir is new for them, and came with Mojave, which is not supported natively, and would be a patched system to even work. And, would also be working within the limitations of only 2 GB of RAM, not upgradeable.
OP can't even reinstall that existing Mojave system, without creating a patched installer for Mojave. Much simpler to just go to an older system. Even El Capitan is challenging to install because Apple, in their wisdom, has never updated the installer to accept current dates on the system. You have to reset the system date manually to install El Capitan. Yosemite (OS X 10.10) is a better choice, as the installer has no such issues. AND, it handles the limited RAM in a 2GB system better than El Capitan does.