Hi Greg.
Conceptually, I think ArcGIS Pro provides all the editing tools you need to create indoor data from scratch. In many ways, you can start creating indoor feature polygons and lines without ArcGIS Indoors license. ArcGIS Indoors (license) enables you to toggle between the different floor levels and see what's on each floor.
If you can start to measure the exterior building dimension and know the XYZ coordinates, that would be the start to creating a building footprint.
Once you have that, I think you can start from the door location and measure and design the corridor/hallway passages first.
If CAD/BIM is unavailable, then I'd even look into PDF or JPEG drawings and start there. You can georeference those image files, use them as reference images, and also start to digitize/trace.
I'd recommend starting with understanding the ArcGIS Indoors Information Geodatabase's feature dataset, feature classes, and schemas are structured.
https://pro.arcgis.com/en/pro-app/latest/help/data/indoors/arcgis-indoors-information-model.htm
Video of floor-aware widget
https://www.youtube.com/watch?v=u7_FSg4sAVQ
If you need to know the editing capabilities of the ArcGIS Pro, trips and tricks, etc, I'd recommend taking a refresher course or reviewing YouTube videos that are already out there.
https://www.youtube.com/watch?v=oprEyMCKyug
Once you have those room or building dimensions, then it's just a matter of using the right editing tools at your disposal. I'd imagine direction, distance, and elevation commands in Pro's editing tool will be very handy.
https://pro.arcgis.com/en/pro-app/latest/help/editing/direction-distance-and-elevation-commands.htm
