What Is Their Main Difference?
'Someone' refers to a person without saying exactly who they are. 'Someone' is more formal than 'somebody'. It refers to a person who is not known or mentioned by name.
'Some one' refers to one unspecific person in a group of things or people that we are choosing from.
Remember that almost all the time, when we use 'some one' it has to be followed by 'of + noun (phrase)'.
When it is not followed by a noun phrase, it's written as 'someone'.
Can We Say 'Some one of the ...?'
Usually, 'some one of the ....' is not used in written or spoken language. We often use 'one of the...'.
'Some' in this usage is usually omitted.