INSIDE has the same meaning as IN,
but you can use INSIDE
when talking about enclosed places
or when emphasising the location: the inner part of something.
– It’s inside the box. (I don’t see it, because it’s ‘deep in the box’ or the box is closed)
– It’s in the box. (maybe I see it, maybe the box is not closed).
INSIDE is also a noun:
Watermelons are red on the INSIDE and green on the OUTSIDE.
INTO is different in meaning, because it expresses direction.
Imagine a dog running towards a lake and when he’s there, he runs INTO the water.
I live in the suburbs, but sometimes I take a train to GO INTO town to meet my friends. I go to the inner part of the town, I GO INTO the centre.