With boot disks. When installing an OS, it was common to have the installer ask if you wanted to create a boot disk in case anything happened to the MBR. They also came with the OS if you bought it prepackaged.
There was also a trick that would boot a Linux system from DOS using loadlin.
I’m always a bit amazed of how things have progressed and on what Linux can still run.
This is an extreme example, but it’s also possible to run a modern Linux OS on SBCs like a Raspberry Pi Zero, and still have something somewhat usable depending on your needs.
To have a computer half the size of a credit card with more RAM than my full tower rig from 2001 is amazing. And it can even run software from that era with dosbox or wine.
My 15 years old laptop is still supported and can still read 1080p on YouTube, using Linux.
Linux devs just recently decided to drop support for 486 CPUs and some early Pentiums.
There’s just no competition.