My question is very similar to Publishing Geoprocessing Service to ArcGIS Online, and is almost but not completely answered by Brad Posthumus as the top answer. Following from that, basically I want to know if a geoprocessing service that has already been published to ArcGIS Server (providing a REST URL) can be consumed by/added to an ArcGIS Online web map or app. According to Esri, a "geoprocessing service (URL)" can be "use[d] in web [and desktop] apps", but it's not clear if that includes ArcGIS Online web apps. I can't find an option in AGOL web map or web app builder that allows you to add these. At Use geoprocessing services from ArcGIS Online in Esri's documentation it says "using the REST URL specified in ArcGIS Online, you can incorporate geoprocessing functionality in your web applications" but doesn't elaborate.
My previous experience has been with authoring and sharing GP tools to other desktop GIS users via custom toolboxes. I'm trying to do something similar in AGOL for non-GIS users.
To be clear, I'm not asking about the Portal/web tools/GP widget workflow, this is specifically about AGOL.