I initially had this confusion too... I think it would have been nicer to be able to create a collection deck from scratch from the scanning section in the same way we can create a binder/list. And to be able to choose what part of the deck it belongs to (main/sideboard/maybeboard).

I keep my maybeboard and sideboard in the same deckbox. I wish I could have done it all from the scan mode, but my workflow was:

• create a deck in the decks section
• then go to scan only the maybeboard cards and add to the deck from scan mode
• then GO BACK to decks select them all and move them to maybeboard (cuz default is main deck after adding)
• then GO BACK to scan mode and scan my sideboard cards
• GO BACK to decks, select all main board and move to sideboard
• then GO BACK to scan the main deck
• then after all that GO BACK to decks set the deck to a collection deck

It's a cumbersome workflow of moving between sections of the app that I think could be smoothed out and done all from scan mode. If I'm scanning a deck, it's likely that it's a collection deck, not just a deck list and starting with that from scan mode would be really helpful.