It's a weird variant of TOTP, but you have to be rooted or modify the app in order to extract the secret to use with other apps. Years ago I wrote a script to do it, but I'm not sure if it still works -- it's not really worth doing imo
The issue is that the app uses a custom protocol to confirm Steam community transactions (Steam inventory trades, etc). So if you use an authenticator like AndOTP, you lose the ability to confirm those.
I reverted to e-mail. I only have free software on my phone, and don't regret that choice.
https://github.com/steamguard-totp/steamguard-shared-secret