How would I connect digital technology to this old Zenith TV?

[Electronic M]

You plug the RF directly into the rear jack marked VHF, UHF, CATV.
You ever hook up a VCR or a 90s game console with only RF output (like a Sega Genesis) to a TV before?... It's the same concept. Heck you can use a VCR as a AV to RF modulator (just have to set the VCRs TV/VCR RF switch to VCR mode and tune or switch the VCR to it's AV input).

Most modulators do not transmit, and instead have to be connected to the TV by wire.

If you need to transmit (I do because wiring 100 TV in 3 rooms on 2 floors is impractical compared to antennas) and are okay spending a few times more than a wired modulator costs look for a Blonder Tongue AM60 or BAVMz (on this one make sure to find one that matches a blank channel in your area) family modulator.