The Librem 5 comes with PureOS by default. This page explains why that is important. An operating system in your best interests The Librem 5 comes with PureOS by default, not Android. It is the only smartphone on the market today to ship with a fully-free and auditable operating system, from the ground up, that …
PureOS is not compatible with Android or Windows smartphones. There are several reasons for this:
These devices cannot work without proprietary parts that PureOS does not ship;
These devices will not work with the latest mainline kernel (system’s core) found in PureOS; they need older versions with proprietary parts;
Even if you have a device that does not fall under the previous issues, there is still a decent amount of work needed to make it possible to install and start on those devices
https://docs.puri.sm/Software/PureOS/Overview/Compatibility.html#android-windows-phones
For those who don't want to click through:
PureOS is not compatible with Android or Windows smartphones. There are several reasons for this:
These devices cannot work without proprietary parts that PureOS does not ship;
These devices will not work with the latest mainline kernel (system’s core) found in PureOS; they need older versions with proprietary parts;
Even if you have a device that does not fall under the previous issues, there is still a decent amount of work needed to make it possible to install and start on those devices