Most states give multiple options: you can mail it back, drop it off in person, typically at an election office; or put it in a drop box. If you're mailing it back, do so as soon as possible in case of delays.
This depends on the state, but many of them have trackers to see if officials have received your mail-in ballot. You may also be able to contact your county clerk to confirm receipt.
It may be, but not necessarily. Some jurisdictions are creating new polling sites — including at stadiums — to facilitate social distancing and are closing other sites. Check on your state's voting site before election day to ensure you know where to go.