If you are ready to use the SharePoint Migration Tool (SPMT), which is a Microsoft developed migration tool available at no cost to do the migration, Alerts and designer workflows and Nintex workflows are not supported.
You need to recrate them in SharePoint online.
About the user migration, when migrating your data from either a local file share or an on-premises SharePoint Server document library, a default user mapping file is used. You can also create your own using the following guidelines. Use any text editor, or an application like Excel, to create the CSV file.
If you want to use third party tool such as ShareGate, please refer to below articles:
For professional information about the third party tool, you can ask the third party support for help:
More information for your reference: