Interac doesn't have a monopoly in Canada; Visa / MC obviously have a presence as well.

Are there Visa and MasterCard Debitcards there? From what I saw, and again only a basic look, Interac is the only major player for Debit Cards. Of course, I welcome more competition if Interac were to enter the USA market.

Yes, and in some cases both, e.g. Scotiabank has debit cards that allow both Visa and Interac transactions: https://www.scotiabank.com/ca/en/personal/ways-to-bank/debit...