I recently got fiber installed after waiting ~9mo in preorder. And it was worth the wait!
Assuming your preorder is for Sonic's own fiber network...
1. You can preorder to show interest in the fiber deployment. You also get monthly updates to its development status. But from another thread, you only get updates if your area has already gone through the engineering phase. After fiber is deployed and everything is ready to go, they'll send you an email to schedule the install.
2. No, Do not buy a modem/router. The ONT will be included free with your fiber install. It's the equivalent of a modem.
A wifi router will also be included for $9.50/mo (or Eero router + beacon for $12). You also get a $10/mo discount for the first year, so basically the router is free. If you return the modem within the first year, you'll lose the $10 discount.
I recommend getting the Eero for $12. After the year is up, then it's up to you whether you want to continue to rent or get your own wifi router or wifi mesh kit. If you have no need for wifi or additional wired connections, you can plug your sole PC straight into the ONT.
Another user posted a very detailed explanation of the entire process. His timeframes may not match yours as every deployment is different.viewtopic.php?f=10&t=9554