Ugrading to a new iMac?
All the apps which you have previously purchased, and which are running on your present iMac, are available in the App Store for re-download. However, the best method for transferring your present user account(s) and apps to your new iMac is through Time Machine Migration Assistant, which will be one of the first options when you set up the new iMac out of the box. Make sure you have a current backup of your present machine and that the Time Machine device itself (External USB drive, etc) can be physically connected to the new iMac.

Just recently I upgraded to a 2019 iMac myself, and because I have loads of Thunderbolt drives, including my main Time Machine drive, I had to purchase Apple Thunderbolt > USBC adapters to do the migration and continue to use the drive as the Time Machine backup disk.