Open source, google play services, and apk for dummies

Hi I’m 45 years old and have been trying to teach myself about open source, google play services and apk for the past three years and I’m getting no where. Anytime I ask family are friends they have no idea of what I’m talking about are they don’t know any more than I do. This is very important to me that I learn how to use these. I’m begging someone out there reading this post please help me!! On open source I know that you see everything about the page even the hidden stuff right? Ok need help understanding how google play services for instant apps work? And lastly apk android package! Please lmk if I’m correct and maybe further explain. With apk downloaded you can literally do anything you want say you download Facebook apk. Would I be able to have a Facebook that some people could see if they had my password and went on my page would things be hidden from them. How does all of this work. Please for my sanity and the sake of my children someone please help