They can be the same in some cases, but "onto" is a much more specific. We use "onto" to indicate mounting something (e.g., a horse or a table) or entering a large means of transport (e.g., a plane, train or ship). We can also use "on" in all of these instances (we can get onto a plane or get on a plane). However, we can only use "on" for many other activities (e.g, we can lay on the floor, but we can't lay onto a floor). And we also use "on" as a preposition in many different ways where "onto" cannot be used (e.g., on Friday, on average, on account of, on schedule, etc.).

onto = change of state from not being on to being on. on = no change in state (see above) BTW, "onto" is usually abbreviated to "on". The same with "in" and "into". This applies to prepositions only. As particles of phrasal verbs we use "on" to mean a change of state. What a language!