OLD AIRPORT USED FOR STORAGE?

As MrHoffman has pointed out, you can certainly use your Time Capsule for a file server services. Just be aware of three things:

1. The TC is not a NAS. It has very basic file sharing services.
2. The TC is limited to either AFP or SMB v1.0 for the file sharing protocol communications. That would mean, if you need the access the TC with a Windows 10 device, you will find that you would have to reconfigure that device to use SMB v1.0. This would be a downgrade and potentially compromise security. Not necessarily a bad thing on the local network, but if you have plans to try accessing the TC remotely, you should be aware of that.
3. Lastly, file transfer performance will be dismal. That is because the TC's CPU is limited when it comes to its file server functions.

... and how would we "get it" when you didn't provide that information in your initial post. We don't have ESP. We can only go on what little information you provided.

In that case, your TC would connect to the Xfinity router by Ethernet, and as such, it would need to both be reconfigured for:

1. Placing it into Bridge mode, and
2. Disabling its wireless radios.

Both of these would require that you use the AirPort Utility.

You have an AirPort Time Capsule with two terabytes of storage.

You wish to connect that to your network, and use that device for network-attached storage (NAS).

To do that, disable the Wi-Fi on the Time Capsule, and enable and configure the file server in the Time Capsule as was described earlier, and wire the Time Capsule to your Ethernet network.

I’d be looking at a Synology or other NAS with more storage, though. Two terabytes is smaller than it used to be.

I’m quite familiar with this configuration, having set it up multiple times. I’m using these as Time Machine targets pending Synology replacements, but NAS is enabled. If you have questions past what’s in the previously-linked docs, please do post a little more about your question or concerns or confusion or local considerations.

Now if you are asking about remote access to this storage, you’re headed for some wrinkles around access security. But connecting local NAS to the router or a local Ethernet switch is typical for these configurations.

Turn off the Wi-Fi, and configure the AirPort Time Capsule file server. This using AirPort Utility.

https://support.apple.com/guide/aputility/share-a-usb-hard-disk-on-your-network-aprt2703/mac