2.5 gig

If one of the ports has an orange or red stripe on it, that is supposed to be a 2.5Gbps port.  Hardware will auto-negotiate the speed if you have it connected to another 2.5Gbps port with a decent cable.   Cat 6 can do it, but Cat 7 is a better cable. 

In bridge mode, only the one Ethernet port will be active on Xfinity stuff.  There is no setting for speed either in bridge or router mode.  The hardware will do it by itself if it can negotiate the speed with the port it's connected to.

Can you actually get 1.2Gbps?  Meh, it depends on your location, cable distribution there, condition of the lines in your area and what they are connected to, congestion in your area.  Up to speed = theoretical calculation based on the DOCSIS modems and aggregated channels.  Your millage may vary.  If you actually need consistent bandwidth that fast (for some reason -- multiple frequent high bandwidth users/multiple users server application/small office), go with fiber. 

You just need 2.5Gb capable devices to connect to the 2.5Gb port.  It's fairly common on new mainboards but pretty hit & miss with laptops and "business" desktops.  Probably mostly miss.  You can, however, get PCI-e 1x add-in cards for desktops and USB adapters that support 2.5Gb.  I have a $30 Asus USB adapter and my 2019 ThinkBook 14s laptop gets full speed off my 1.2Gb Comcast connection and 2.4Gb on a local speed test with it.  The computer matters though.  My work issued machine is older and the USB adapter doesn't work well with it.  I get better speed off the built-in 1Gb port on that old thing.

You don't need your own router to get 2.5Gb to wired devices.  You can just plug a 2.5Gb (or faster) switch into the XB7 to support >1 device at that speed.  If you want 1.2Gb to WiFi devices you need a 160Mhz wide channel on the 5GHz or 6Ghz band.  Some WiFi 5/802.11ac devices can do it under ideal conditions.  WiFi 6 or 6e (802.11ax, 6e = 6GHz support) would be better.  I've seen conflicting reports on Xfinity gateways supporting 160Ghz channels on 5GHz, and you need an XB8 for 6GHz.

2.5Gb works on Cat5e or better.  Better cable just gets you a longer run.  Also you don't want real Cat7.  Cat7 technically uses a weird connector, but a fair number of vendors have taken to marketing cabling as Cat7 since it's a more or less abandoned proprietary standard.  Use Cat6a or Cat8 if you actually need shielded cable.

You probably shouldn't bother thinking about fiber.  I'm running a mix of Cat6, fiber, and DAC cables and I can't see why you need fiber at home for performance or cable length reasons unless you have a huge house or you're out on a farm and trying to run Internet to outbuildings.  Cost and lighting protection are a different story.  You know how cable companies tell you not to use a surge protector on the coaxial?  You can set up something much better than a surge protector between a modem and router or a gateway and switch with fiber for ~$100 or a bit less on a 1Gb network.  On the cost side it depends on what you want to do, but generally the things that make fiber at home cheaper are wanting to play with server equipment and buying used stuff on eBay.  If you have the technical chops to deal with used datacenter gear you can get a 10Gb fiber network between higher end desktop & server computers for less than a 2.5Gb network.  You need higher spec computers for that to work though.  They don't need to be new (I have a 10yo one that does just fine on 10Gb), but they need to have extra slots to accommodate stuffing used server NICs into them and a 1x PCI-e slot won't do.  A lot of desktops just have a 16x slot for a vid card and a couple 1x slots.  You need at least a 4x for a used server NIC and an 8x would be better.  Also don't forget about the cooling because serer NICs expect a bit of airflow like they'd get in a rackmount server chassis.

@marksllc 

In short, the 2.5Gb port worked on my XB7.   I purchased a router with 2.5 GB WAN port and connected it to the port on the XB7 with a red stripe on the side of it. Once I restarted the router and XB7 they connected at 2.5 Gbps.