well-organized OR organized well? / on their own OR by them'selves?

What is the difference between these two sentences:

They are organized well.
They are well-organized.

I think the former is informal. That is right?
Are there any other differences?

Also, I would like to know about the difference of these sentences:

They can do that on their own.
They can do that by them’selves.

They are organised well. - Someone (possibly themselves) has organised them

They are well organised. - Usually, they know what they are doing and are confident and efficient.

Both are equally formal.

They can do that on their own. / They can do that by themselves. (Note there is no apostrophe)
Both of these mean the same.

Thank you very much Beeesneees.