Well I've been around computers since the late 80's(I was like 3 actually). Went the whole mile: Atari Xl-XE, MS-DOS, Windows 3.1, 95, etc. Then started using Linux because of the looks(yes I know, Compiz-Fusion was the real reason to explore Linux) and now is installed in all my machines. I've even have it in Windows 10.
I've assembled machines from scratch before and you could always boot to "MS-DOS", that is what I remember and that made me wonder.
How was Unix installed back in the 80's or late 70's(I wasn't even alive)? Was it trivial like boot a big floppy or black magic involved?
It happens that I can't find any references to it and people in my country just don't get Free Software thing.
EDIT:
I've skipped a crucial part, I feel dumb because I didn't think about that in the first place.
Eveything starts with the boot sequence, which isn't an Operating System, but it's in ROM memory, like the BIOS(In my mind is a very minimalistic OS for machine config) and other stuff, like the boot sequence. At this stage it will look for the devices listed in the BIOS then it will iterate them in order, till one device responds with the boot instructions, like the ones in the tapes. So no initial OS is necesary and Unix can be installed.
Dumb mistakes, takes you the long way, but surely you learn more.
Best Answer
In the first job I had, back in 1989, we installed Unix (Interactive, owned by Kodak at the time) onto Intel 386 PCs using 3.5 inch (ca. 9 cm) floppy disks.