This page will explain how to add a feature layer from a file to a Collector web map in ArcGIS Online (AGOL)
In AGOL, navigate to your Group page
Press the "Content" tab in your Group
Find and select your Collector Map (Organization Name)
Press the "Open in Map Viewer" button
See the Map Viewer with the Collector Map (Organization Name)
Press the "Add" button
Choose "Add Layer From File" Note: There are different options available, including, from layers
Press the "Choose File" button. Note the rules for files in the splash screen
Press the "Import Layer" button
Symbolize, or stylize, the layer
Press the "Save" button