Into means going inside
Into is a preposition
I went into the house.

In to has no meaning by itself
In is an adverb and to is a preposition
He gave in to their demands.

Onto means upon
Onto is a preposition
I jumped onto the platform.

On to has no meaning by itself
On is an adverb and to is a preposition
He held on to his hat during the gust of wind.