How good is the apps business? Anyone can share their experience?

Came across many youtube videos suggesting apps for lucrative earnings
But a wise man will always trust BHW members >>>>>> youtube clickbait

How much simple games apps usually make?
Are they worth while for long term investments?

If you make a good product you're going to get good money. A game has to be addictive in order to earn from it. You can put ads in game or membership for removing the ads (like 10$ per month). To calculate the ROI just calculate how many ads you can get away showing to free users and calculate the income from the ads.