Friday's deadline wasn't particularly significant. It's only significance to for scheduling future arbitration hearings. It's also common for players to sign multi-year contract extensions while awaiting an arbitration hearing. If the case goes as far as the arbitrator, however, the player's salary will be either what he demanded, or what the club offered, and always a one year contract.
The arbitration cases are not necessarily all held before the season starts. Last year, Aaron Judge avoided arbitration with the Yankees by signing a one year contract on June 24. In a case like Bichette's, his regular season pay prior to an arbitration hearing would be based on the team's offer, (ie: the minimum salary), and the difference would be paid to him if he won his arbitration case, if it comes to that.
Arbitration cases often create 'bad blood' relationships between players and their team. It's like a company performance review, by a manager who doesn't like you personally, or doesn't want to give you a raise.