Actually, the early ARPAnet was mostly DEC PDP-10 machines.[1] MIT, CMU, and Stanford were all on PDP-10 class machines. Xerox PARC built their own PDP-10 clone so they could get on. To actually be on the ARPANET, you had to have a computer with a special interface which talked to a nearby IMP node. Those were custom hardware for each brand of computer, and there were only a few types.
The long lines of the original ARPAnet were leased by the Defense Communications Agency on behalf of DARPA. ARPAnet was entirely owned by the Department of Defense, which had
no problem renting a few more lines from AT&T.
AT&T was willing to lease point to point data lines to anybody. They were not cheap. I had to arrange for one from Palo Alto CA to Dearborn MI in the early 1980s, and it was very expensive.
The long lines of the original ARPAnet were leased by the Defense Communications Agency on behalf of DARPA. ARPAnet was entirely owned by the Department of Defense, which had no problem renting a few more lines from AT&T.
AT&T was willing to lease point to point data lines to anybody. They were not cheap. I had to arrange for one from Palo Alto CA to Dearborn MI in the early 1980s, and it was very expensive.
Here's a good historical overview.
[1] https://www.visualcapitalist.com/wp-content/uploads/2019/03/...
[2] https://www.researchgate.net/publication/329976721_ARPANET_a...