Is it possible to add a Geojson file from a server or cloud (Google drive for example) into an ArcGIS online map?
I can add Geojson files hosted in my computer without any problem, but when providing the url from a Geojson file hosted in Google Drive, it doesn't work.
To my knowledge, this isn't possible in the way you are trying. As you noticed, it is possible by uploading a file.
If this was ArcGIS Enterprise and you had the GeoEvent extension then you can read GeoJSON events from GeoRSS and convert them to feature service features.
Alternatively, you could write your own parser, polling the GeoRSS feed and converting the features yourself. Once converted you can add them to a Feature service with applyEdits.