Hello! You can use 'in' when, yes, 'you are in a house' like a physical object. Another way you could use it would be when you are experiencing a situation. For example when 'you are in trouble' it means that you have done something wrong, which had gotten you to be scrutinized for your actions. On your second point of 'being on the beach'. In this case it would be in the environment. It means that you are surrounded by that environment. For example saying that you are 'on the beach' would mean that you are in that environment of the beach. I hope this answer was clear and helped you!

Perhaps you should use images instead of words. Type this into your browser's address bar: "prepositions with pictures"

You can think of it like this: 1) when something is completely enclosed by something much larger -> "in" 2) when something is enclosed partially, or in certain dimension only -> "on" You are "in" a house since a house encloses you in every direction. The beach is much larger than you horizontally, but not vertically as you stand on it. So "on". You are "in" a city, in a province, in a country, and in the world, as they are all big and surrounds you in pretty much every direction. But you are on a river, on the ocean, and on the ground since they have clearly defined surface on which you stand or float. Also "in" and "on" is not always fixed. Wile you can be on the ground, you can be "in" the ground too. For example if someone dies and is buried, they are said to be "in" the ground. So instead of just memorizing set phrases, one should acquire the sense of it.