Operating Systems for Libre Computers are all Linux based. Operating systems come in two categories, first party and third party. First party operating systems are maintained by the Libre Computer Project while third party operating systems are provided by various groups not affiliated with the Libre Computer.
BTW, I come from Raspberry Pi and I’m a bit lost.
Things are quite confusing. For example, there’s no recommended image. I’ve tried with Raspbian, because I was used to it. But it feels a bit weird because it looks like it’s not “proper” to the hardware. Also, I was bitten by this issue: Signatures were invalid: EXPKEYSIG 2E5FB7FC58C58FFB - #5 by SuperJMN
I hope you can make newcomers’ experience a bit warmer.