A video app can wait for Wi-Fi, while your maps still need mobile data on the way home. Use the app’s own switch in iPhone cellular settings. Leave the main Cellular Data switch on if the rest of the phone still needs that connection.
Find the app’s switch
Open Settings → Cellular or Mobile Data. Scroll to the apps and services, and use Show All if the list is collapsed. Turn off the switch for the intended app. Apple explains that the app then uses Wi-Fi for data. Check the name before switching it off, especially when two apps have similar icons or names.
Verify with something small
When convenient, try opening a harmless new item in that app with Wi-Fi off, then reconnect to Wi-Fi and try again. A page already downloaded can open from the device, so use an item you have not opened. Restore Wi-Fi after the check and leave other app settings alone. If the app needs to load new content away from Wi-Fi later, turn its switch back on. Look to your carrier for the actual billable usage period.