How do I merge all my devices so when I input on one device shows on the others?

Merging is not the term I would use for this.

This is what would be called syncing. This is done by using the same Apple ID for iCloud on each device.

When you do this, you can selectively sync various content in each device's settings. When elected, that content type is synced to iCloud and then to each device separately.

So the devices do not directly talk to each other, but talk to iCloud and sync that way.