Wouldn't they have arrested me then and there
They could have, but they didn't.
They can call the police and report the crime. The police can file an application for criminal complaint in the courthouse having jurisdiction over the city/town where the bank is located.
Assuming the clerk finds probable cause, a summons or warrant will issue. A summons will be mailed to your house with a date certain to appear before the court. A warrant may or may not be mailed to your house, but the police may show up with the warrant in hand before the mail is delivered.
If the police find you they will arrest you in the straight warrant and brought before the court to be arraigned. If the police that arrest you are not within the geographical jurisdiction of the court that issued the warrant, you may be held for transport to the court that issued the warrant. So if you get arrested Thursday evening, you'll spend the night in the police station lockup, brought to court Friday morning, and held for transport to the court that issued the warrant. That won't happen the same day, so you'll spend the weekend in the county jail and will be brought to court Monday morning (assuming Monday is not a holiday).
If you learn there is a warrant issued, you should consider high tailing it to the court that issued the warrant to avoid being locked up.
Once you get to court, you'll first talk to probation to see if you qualify for a court appointed attorney. If you're locked up, they'll come find you.
Once in front of the judge, the prosecutor may or may not ask for a bail or other bail conditions. If the prosecutor is asking for any amount of bail, the bar advocate will argue on your behalf. This is not the time to argue the merits of your case. This is a bail hearing only.
If you qualify for a court appointed attorney, one will be appointed for a fee. The fee is $150 if your income is below the poverty level, or $300 if under 125% of the poverty level.
Larceny over $250 is a felony and carries a maximum sentence of state prison for five years. In order to sentence you to that, the DA would have to indict you and bind you over to the Superior Court.
If the District Court retains jurisdiction, the maximum penalty is two years in jail.
What the DA will offer for a plea depends on the county you are in. If you are in Worcester County with no record, I suspect your attorney could plead it down to larceny under $250 (misdemeanor) and get straight probation. Middlesex county likes to send people to jail.