@OntheMain
First of all, a lot of people new to linux get frustrated quickly when it’s not exactly what they’re used to with windows, so thank you for asking politely
You won’t find a Qubes .deb package. Qubes is a totally different “distribution” from PureOS. In windows there are only a few “distributions”, like windows 8, windows 10, windows mobile, etc…
Since linux is open-source, there are thousands of distributions, and any teenager could make a new one in their basement over the weekend. They all share the linux kernel - the fundamental “core” of linux - but different distributions can be wildly different.
Since Qubes is a different distribution, you will need to get the Qubes .iso file and burn it to a USB stick. Then, boot from the USB stick, and install Qubes as your new OS.
.deb files are software packages that you can only install on distros that use the debian package management system. Ubuntu, PureOS and Linux Mint are examples of linux distros that would accept .deb files.
.iso files are full disk images, that you have to copy to a USB drive or DVD, and then you can tell your computer to boot off the disk image
You can’t just drag and drop the Qubes .iso file to the usb stick though. You will need to use a program, like unetbootin, to make the usb stick bootable.
P.S. Since Qubes is based on Red Hat, their package files will end in .rpm
P.P.S I haven’t used Qubes or the Librem yet myself (in the mail yay) so I can’t help too much with specifics