In the sentence  'Some day I am going to . . '  or  'One day I am going to . . ', there is no difference; they mean the same, that you intend to do something.

Someday and one day mean the same.

some day=>يوما ما

one day:

one=>واحد

day=>يوم

one day=>يوم واحد

 someday =   یوما من الایام  او یوما

 one day= یوما

The first thing to say is that one day is written as two words, except when used as an adjective, when it can occur as one-day. Some day is also written as two words, except when, as in your example, it is an adverb (although even then you might still find it as two words).

The choice between the two has little to do with the timeframe. In saying One day, I will be a doctor, the speaker shows a degree of determination and certainty which is lacking in the vaguer Someday, I will be a doctor