The straight answer is a modem bridges and translates between layers in the OSI model, typically layers 1&2 (physical and data) on one side (your coax for cable or fiber) and layers 3&4 (IP and TCP, the typical protocols for home networking) on the other side. We used to call modems the devices that would do that translation over a phone line, and we would call the devices that connected two different network types bridges (like coax or microwaves to Ethernet or token ring), but now we call what are really bridges modems for home networking deployments.
To get between ELI5 and PhD, dig into some good telecom textbooks and focus on the fundamentals like the OSI model, time-division multiplexing (TDM), frequency-division multiplexing (FDM), DOCSIS, how to send binary data over radio, etc.
Someone already mentioned this below but it’s worth calling out: Comcast drastically limits the upload speeds when you bring your own router. It’s terrible practice and it punishes advanced users but is what it is. So if your dad is currently utilizing faster upload speeds for work, then he can expect those to go down when you switch him to non-Comcast gear.
I was told by a Comcast rep recently that this policy is going away (at least where I live) by the end of ‘24 so bring your own gear will get the same upload speeds as their gear, but we’ll see if that happens.