I have multiple users on my old Mac. Will migration assistant move their information, setup and applications? Or, do I need to run the Migration Assistant for all users

It will migrate all users. When you execute it please follow the steps in Setup Assistant and Migration Assistant, why... simple, it is the fastest and most reliable method and works 100% of the time assuming your Time Machine External Hard Disk is functioning well.

I believe it's one user at a time, you need to be in the account to open migration assistant, if not you could migrate someones admin account without their permission.

Personally I'd migrate set up the new computer with the apps you need, patch them, then migrate your data per user, not the apps if possible. I've done a number of single migrations over the years for end users and this seems to be the most stable procedure for me when I'm finished.

To add to the reply by @ rkaufmann87, when you setup your new Mac and before you use Migration Assistant, it's best to create an admin account with a name unique from the accounts you plan to migrate, and then login to that account and run MA.

If you already have created a user account on the new Mac that has the same name as a user account that will be migrated, then you're going to find what appears to be a duplicate account (like "User" and "User-1") after migration, and merging those accounts and user data can be a little bit of a chore if you've already begun to use it.