I think most Mac people use some form of iCloud Storage plan. I use the 200 GB maximum size which works for me, but many people use lesser amounts. You can upgrade or downgrade your iCloud Storage on the Mac and in iOS. Here is how that is done.
Mac iCloud Storage
On the Mac you change your iCloud Storage in your Apple ID account in System Preferences.
In the next window click on the “Manage” button.
In the Manage area click on “Change Storage Plan”.
I have the maximum plan, so I can only downgrade. However, if you have a lower storage plan that button will say “Upgrade”. When you are in the next window you can adjust your plan, that is all there is to it.
iOS iCloud Storage
You have to go into Settings to change your iCloud Storage in iOS. Once in Settings, tap on your Apple ID account.
In the next window tap on “iCloud”.
In this screen tap on “Manage Storage” to get into the adjustment area.
Finally, tap on “Change Storage Plan” and upgrade or downgrade your plan.
Changing your iCloud Storage is pretty easy. I have never downgraded mine, but I did eventually upgrade to maximum setting.