How you can see the time you have spent on mobile apps in Samsung Galaxy A20 and also how to set timer for mobile apps?
First of all, I want to say thanks that Samsung has provided this useful feature in almost all Samsung mobile phones. This feature is really helpful to those users who want time management in their life.
How to open it?
First of all, Go to your phone Setting
Digital Wellbeing and Parental Control 
Then you can see How much time you have spent on mobile apps. Here you can also see the number of notifications came into your applications such as WhatsApp, Messages, etc. Here You can also set timings or time limit to your applications so that if you will use more time than your set time than it will directly close your app and alert you that you have used more than set time.
But if have some emergency and you must to use that application then go to setting , then digital welbeing and change your custom timing as per you compatibility.
Below the image screenshot is given for your better understanding:
