How can I Automate for selected apps to only use cellular data and not Wi-Fi... and vise versa?
On the Apps section, select App in foreground? Tap on the block. On the package option, select your app. Press Save.
On the Connectivity section, select Wi-Fi set state block. Set the Wi-Fi option to On. Press Save. Link the block to the Yes output of Step 1.
On the Connectivity section, select Mobile data set state block. Set the Mobile data option to On. Press Save. Link its input to the No output of Step 1.
On the Connectivity section, select Wi-Fi set state block. Set the Wi-Fi option to Off. Press Save. Link its input to the output of Step 3. Then link its output to the input of Step 1
Your flow will look like the image below. Give it a name, give the necessary permissions, start the flow.