Drop Box
How to use across iPhone, iPad & desktop
iMac 27″, 10.15
You need to install the DropBox app on all your devices and use the same DropBox login ID and password on all of your devices.
Within apps that support backups to other online, offsite data storage services, like DropBox, you need to learn/find out how to set those apps up to backup their data to your DropBox account.
You may have to go the partcular app/s support Website/Webpages to learn how to do this.
Best of Luck to You!