What modifications are you looking to make?
The Solution Deployment Tool lets you modify domains and add fields. You can also easily delete fields from the Details page of the feature layer items.
More extensive modifications can be made by exporting the layers to a file gdb, opening them in Pro, and overwriting the existing layer with your updated schema, although this may break some maps and apps if you remove fields that are used in popup configurations, etc.
If you just want to see the schema of the layer, you can click the Service URL link on the Details page for the layer to open the REST endpoint which will show you the fields, templates, etc that are saved as part of that layer.
