Both the app and PC software must be signed into the same account, and the routes you want to share must be in a cloud-synced layer. That’s the only kind of layer you can create on the mobile app, but Coastal Explorer on PC might need you to import an .NOB file or copy/paste routes into a cloud-synced layer you’ve already created.
If you’re unfamiliar with the cloud-synced layer features, here’s a video on the topic from the “Help, Tips, FAQs, and Videos” forum category:
There are several other useful videos and articles in there as well.