Onto means moving to a place on something, usually after a movement verb.
The cat jumped onto the table. The cat starts on the floor and lands on the table, so onto shows the movement to a position on the table.
That landing-place idea works in real scenes you already know. Picture someone walking out onto the stage: their heart may be pounding, but the word itself only records the path their feet take, from the wings to the boards under the lights. Or picture clothes tumbling out of a suitcase: onto follows them from inside the case to the heap they make on the floor. In each scene a verb of movement comes first, and onto then answers the question where did it land?
She emptied the suitcase onto the floor. The clothes travel from inside the case to a heap on the floor, and onto names that landing place.
The sheep were loaded onto trucks. Each animal moves from the pen up onto the trucks, so onto marks where it ends up.
Onto is about movement, so it fits sentences with verbs like jump, climb, or step. When nothing moves, plain on is the usual choice.
The book is on the table describes where it lies. She put the book onto the table describes the motion from her hands to the surface.