Ruby Native needs three things from your Apple Developer account to sign your app and upload it to TestFlight: your Team ID, an App Store Connect API key (with its key ID and issuer ID), and the key's .p8 private key file.
Your Team ID identifies your Apple Developer account. You'll use the same one for all your apps.

If you're on a team with multiple members, any member can view the Team ID. It's not a secret, just an identifier.
The API key lets Ruby Native upload builds to TestFlight on your behalf. You create it in App Store Connect, not the Developer portal.
Make sure it's a Team Key, the kind under Users and Access. An Individual Key, generated from your own user profile, can't access provisioning, so it can't sign your app even with the Admin role.

After generating the key, you'll see it listed in the table. Three values come from this page:
AuthKey_XXXXXXXX.p8.
Important: Apple only lets you download the .p8 file once. If you lose it, you'll need to revoke the key and create a new one. Store it somewhere safe.
Yes, as long as it's a Team Key with the Admin role. An Individual Key can't sign apps, even as Admin. To reuse a key, you'll need the key ID, issuer ID, and the .p8 file you downloaded when you first created it.
You need an Apple Developer Program membership ($99/year) to distribute apps through TestFlight and the App Store. Ruby Native can't build or upload without one. See the enrollment guide for step-by-step instructions.