You can read everything about that train service here: https://www.sncf-connect.com/train/trajet/marseille/aix-en-provence

There are regular regional commuter TER trains so they don’t have to be booked in advance as they are not seat assigned unlike TGVs.

Top tip: Download the SNCF Connect app on your phone. This app has time tables, tickets for purchase and the storing of your purchased ticket all in one accessible place. You have a QR code for these tickets so it’s best to screenshot it and that’s your way around not having internet. Your SNCF train e-ticket doesn’t need to be validated, the inspector just scans the QR code. See: https://www.sncf-connect.com/en-en/tools/mobile-app

The alternative is bus line 50, from the bus station next to St Charles. train station.

Line 50 + runs every 15 minutes via the motorway.

https://www.lecaraixmarseille.com/line-50