You can drag+drop anything you select in Concepts (including multiple strokes) into other supporting apps, like email, text, document editors etc. The gestures for drag+drop are the same in Concepts as for the rest of iOS11+.
To drag+drop selections from Concepts to another app, select your item (can be one stroke or multiple strokes or strokes on multiple layers... whatever you select is game) and tap+hold the selection box. Once the thumbnail appears showing your selection inside it, drag it into the other app. While your item is selected, you can either bring up the command bar by swiping upward from the bottom of the screen, or you can enable a split screen workspace, then drag in your item. The item will be dropped into the other app as a flattened PNG.
If this gesture is getting in the way of your selections, you have the option to turn off this feature. Go to Settings -> Gestures, and under the Tap+Hold section, uncheck the box beside "Enable iOS drag+drop of active selections".
To drag+drop images or other file types from another app (such as Google or email) into Concepts, tap+hold the file of choice and drag it into Concepts. It will appear on the canvas, selected and ready to go. Tap to stick it to the canvas.