The Xfinity ones show up only after you install a profile from the Xfinity app or website, and manually enable it in Settings.
The Xfinity hotspot service typically has two SSIDs going: "xfinitywifi", which is unencrypted and has a captive portal to log in, and the "XFINITY" network, which the installing the profile enables access to and which is encrypted with 802.1X authentication.
For everyone except apparently the carriers, Apple is very strict about the opt in nature of managed WiFi networks.
I have devices with 0 Comcast apps and 0 profiles but they all have the Xfinity managed networks, the only thing they have in common is the cellular service.
The Xfinity hotspot service typically has two SSIDs going: "xfinitywifi", which is unencrypted and has a captive portal to log in, and the "XFINITY" network, which the installing the profile enables access to and which is encrypted with 802.1X authentication.
For everyone except apparently the carriers, Apple is very strict about the opt in nature of managed WiFi networks.