Questions from a beginner who is excited but naive

Hello, I am completely new to investing and using Fidelity and I have some basic questions I’d like help with. I just set up custodial accounts for each of my three young children ($1500, $1000, and $500 respectively). The plan is, obviously, to build them a robust savings upon reaching adulthood.

1. I’ve read in multiple posts how you can put money into a UTMA account and leave it alone and Fidelity automatically invests it for you in a “core” but I’m unclear exactly what that is. When I look at their accounts it just shows everything blank. Do I need to go in and purchase shares in SPAXX myself or will that be done automatically at some point soon? I just set up the accounts two days ago.

2. Grandpa has expressed interest in setting up a monthly contribution from him. Is there a process for him to do so through his bank so he can contribute directly?

3. Lastly, I am completely new to the whole idea of trading and investing but I really want to learn how to start but I have no idea where to begin. Are there resources available that can teach a lay person like myself how to get started and to offer tips on choosing stocks and the ins and outs of everything?

Thank you!