Since nobody directly answered some of your specific questions:
> Lets say I want to boot a random old PC from one of these images - does the bios need to have iPXE support? Or PXE?
The most-desirable way to use iPXE is to boot it from a system's built-in PXE, chainloading it from a local DCHP+TFTP server. All routers have a DHCP server, and many enthusiast/professional routers will have a TFTP server available. pfSense/OPNSense for example can both do this.
If you want to go down the rabbit hole and can mod your system BIOS, you can install iPXE into many of them. I wouldn't recommend it, but I did it once and thought it was pretty cool.
> random old PC
If you _literally_ meant like a vintage PC that predates the PXE spec[1], you can boot iPXE from any local media — I architected a project 10 years ago where I actually gave floppy disks to technicians because some of the ancient Dells we were working with required a BIOS update to enable USB booting.
You can also burn iPXE into a network card's ROM chip. There were a lot of PCI/ISA network cards that took an EEPROM and could boot from them if populated. I always thought it would be fun to boot a vintage PC with iPXE straight off of an add-in network card like that. iPXE will hook the BIOS, and you can literally boot MS-DOS via iSCSI.
> Do I need to have some service running within my LAN first or can this go right out to some public server and boot from images there?
Again, typical deployments will have iPXE chainloaded from the system's existing PXE implementation, which means you've got a DHCP server on the local segment, and a TFTP server somewhere on the local network. But you _could_ boot iPXE from any other boot medium, which would mean the DHCP/TFTP setup are not required. iPXE itself, once loaded, can do its work with any network connection.
> Lets say I want to boot a random old PC from one of these images - does the bios need to have iPXE support? Or PXE?
The most-desirable way to use iPXE is to boot it from a system's built-in PXE, chainloading it from a local DCHP+TFTP server. All routers have a DHCP server, and many enthusiast/professional routers will have a TFTP server available. pfSense/OPNSense for example can both do this.
If you want to go down the rabbit hole and can mod your system BIOS, you can install iPXE into many of them. I wouldn't recommend it, but I did it once and thought it was pretty cool.
> random old PC
If you _literally_ meant like a vintage PC that predates the PXE spec[1], you can boot iPXE from any local media — I architected a project 10 years ago where I actually gave floppy disks to technicians because some of the ancient Dells we were working with required a BIOS update to enable USB booting.
You can also burn iPXE into a network card's ROM chip. There were a lot of PCI/ISA network cards that took an EEPROM and could boot from them if populated. I always thought it would be fun to boot a vintage PC with iPXE straight off of an add-in network card like that. iPXE will hook the BIOS, and you can literally boot MS-DOS via iSCSI.
> Do I need to have some service running within my LAN first or can this go right out to some public server and boot from images there?
Again, typical deployments will have iPXE chainloaded from the system's existing PXE implementation, which means you've got a DHCP server on the local segment, and a TFTP server somewhere on the local network. But you _could_ boot iPXE from any other boot medium, which would mean the DHCP/TFTP setup are not required. iPXE itself, once loaded, can do its work with any network connection.
[1]: https://en.wikipedia.org/wiki/Wired_for_Management - this happened in 1998/1999 and implementation was widespread by 2001 or so.