I know how to find metadata and capture date for basemap imagery in Pro but is there a way to do this with Online/App Builder basemap imagery.
Is it updated regularly or just whenever they have a new image?
There is metadata about when a given area was acquired. In the online realm, you have to add the basemap as a layer to your map and then when you click an area, you'll get a popup with the image acquired date. I'm not aware of a systematic update cycle for the imagery. I think they wait until users clamor for an update in a given area (due to a natural disaster event or significant change etc) or some other beneficial acquisition happens (in the US the USDA acquires new ortho photos for each state on a multi-year rotation).
This works in Pro but not with Online basemaps from the Basemap Gallery.
It may work with basemaps from my own organization added as a layer provided these have that information but not with ESRI's Gallery.
You can obtain metadata for imagery in the World Imagery Basemap. This blog describes how to do that: https://www.esri.com/arcgis-blog/products/arcgis-living-atlas/mapping/world-imagery-metadata-update/
I just realized that blog covered Map Viewer Classic. To view the metadata in Map Viewer, just add the World Imagery layer from Living Atlas, then click on the map after zooming to and in to your area of interest (metadata is scale-dependent).
I just opened a blank AGOL map, searched AGOL for the Imagery basemap & added it to my map and clicked on the map..
Yes, that's the way to do it. Zoom in or out, or pan to different locations and you'll see differences in the metadata.