Hi Emily,
1) I'm currently on ArcGIS Enterprise 10.6.1, do I need to first upgrade to 10.7 before upgrading it to 10.8?
2) Will I need to take the same upgrading steps again when upgrading to 10.8.1?
A1 and A2. No, you can upgrade from 10.6.1 directly to 10.8.
3) When upgrading to Portal, it says "no need to manually Reindex if content is on a file system." How do I know if my content is on a 'file system'?
A3. Where does it say this? In my experience, it is always a good idea and best practice to reindex Portal.
4) If we didn't use the 'Builder' on the last 10.6.1 upgrade (Jump start), can we still use the Builder for the upgrade?
A4. It depends - do you just have a base deployment of ArcGIS Enterprise? Are all the components installed and running on a single machine? or do you have a more complex deployment of ArcGIS Enterprise?
Some resources:
Hope this helps,