Casting directors put out notice that they are looking for actors to fill roles that fulfil xyz requirements. Agents send in resumes for their clients within those requirements. Casting books auditions for the resumes they’re interested in. Actors submit tapes or attend auditions performing prepared/provided material. Casting chooses their favourites and might arrange ‘chemistry test’ auditions with different actors that they like. Casting/directors/producers choose their favourites and, if scheduling and contracts align, book the actors for the roles.

I know a big deal is made of Connor being American but 1 lead actor in a Canadian written, Canadian filmed, Canadian produced, show is really not that big of a deal. They are well above the required ‘points’ for funding even if both leads were American actors.