Your first task is to take a look at the amount of data used by apps on your phone. WiFi Background Data Setting.
It is a tough call – and personal choice – to decide whether or not you wish to restrict the data usage of certain apps. …
How to Restrict Background Data For Individual Apps on Galaxy S10 and S10 Plus One UI 2.0 and 1.1 You can enable or disable app data usage in your latest Samsung Galaxy devices using below given settings.
Find Out Which Apps Are Using Excessive Cellular Data. How do I prevent an app from using my mobile data?
For this you will be accessing the Data Usage section on your Android Phone, which provides a good overview of the amount of Cellular and WiFi data used by apps …
How To Restrict Data Usage by App Samsung Galaxy S10 / S10+ / S10e: Some apps depend upon background data (transferring data when not directly in use).
The steps above will stop Youtube from switching to mobile data unexpectedly so you should be fine, however if you physically open the app while outside the range of your router with Mobile Data enabled, it will utilise the data connection.
From a Home screen, swipe up or down from the center of the display to access the apps screen.
If you open Youtube while there's no WiFi or Mobile Data connection then it just says 'you are not connected to the internet' so there's no danger of it using your data. On my OnePlus 5T I was able to prevent my phone from using mobile data on YouTube so it would only let me stream video's from the app via WiFi. This was great from me as my WiFi often cuts off from the third floor of my house and saved me a nasty data use bill at the end of the month.
Restricting background data may cause those apps to stop functioning unless a Wi-Fi connection is available. (4G, H+ etc.)
Research each one to see what is and is not transmitted when they are not in use, and decide which ones you can safely disable. Use these steps to disable background data on the Samsung Galaxy S10.