This was a general idea back then terminal connections were physical. Small BASIC program (Your system does have BASIC, right?) or hex dump could be typed in to create a lowest common denominator bootstrap communication/encoding/decoding tool, which then could be used to transfer more complex program to the system. On DOS systems you could "copy /b COM1 file" (on a physical keyboard) or "copy /b CON file" (inside the terminal connection), then try to send data as is. If it was small enough, the line was clean enough, the settings matched, and the system was fast enough to chew it with one bite, it could work.

Here's an extreme example of such program consisting only of printable (and type-able) characters:

http://www.columbia.edu/kermit/ftp/a/tcomtxt.asm

(Seen on https://retrocomputing.stackexchange.com/questions/5202/tran... )

You can also notice the mention of bootstrap files in Kermit, and look them up.

I've recently seen some better known YouTube user doing exactly the thing we discuss on an old system (even including the consequences of accidental port setting mismatch), but, of course, can't remember the exact video.

Norton Commander, in addition to sharing files over serial or parallel port, was able to “clone” itself to another computer by sending a bootstrap loader the same way, then copying the rest. Hmm, the null modem cable pin connection chart in the readme seems to be a regular one (though without carrier detect pin), but the interlink/laplink cable is shown as 8 bit wide (though the thing should probably work with popular legacy 4 bit crossover):

https://rentry.co/nc50readme