The "On My Mac" Mail Folder Location

Don't know if this is specific to Big Sur, but here's my question.

At one time I used both an iMac and a Windows Dell laptop (had to do with hobby related software available only on Windows). I started using Thunderbird as an email client because I could locate the "Local Folders" on Dropbox and that way have access to all my stored emails on both platforms. The Dell laptop died a short time ago and I decided not to replace it and just purchased a MacBook Pro with Big Sur installed. Happy with the choice by the way as I purchased my first Apple computer way back in 1984 when they were still called "Macintosh."

I would love to use Apple Mail but can't find any information on where where in Big Sur those folders are located (the iMac is still on Catalina) or if its even possible to store those "On My Mac" Apple Mail folders on Dropbox.

Any help would be appreciated (like could those folders possibly be stored on iCloud rather than Dropbox).