Hi @BK2021
Sometimes the charges for the apps are deducted at the same time when the Shopify Subscription charge is collected, but you can cancel the app subscription at any time and if the amount was deducted in advance, you will get the complete refund back as soon as you cancel the charges and uninstall the app.
So if you want to discontinue any apps you can do it without having a second thought about the charges.
Hope this helps 
Cheers!!