Are both the same? When do we use "fix" and when do we use "repair"
The differences can be subtle. (We'll ignore the meanings of fix that have nothing to do with repair at all.)
At their closest pairing, the two words mean essentially the same thing, the difference being that repair is an "H" word (part of the "high", or more formal, vocabulary) while fix is an "L" word (part of the "low", or less formal, vocabulary). That's the difference that makes a business offer a "repair service" rather than a "fixing service" (or a "mending service", for that matter) even if the people working there would answer What do you do here? with I fix stuff.
At another level, though, the difference is greater. Repairing is pretty much limited to putting things back to the way they were or restoring them to usefulness. Fixing often goes beyond that to include modifications or improvements. One would never speak of repairing something that is brand-new and in perfect working order (it doesn't need to be repaired), but brand-new shiny things can be fixed in order, for instance, to become useful to someone who has a disability that renders the brand-new shiny thing essentially useless to them in practice even though it would meet their needs in theory. You can fix a system that is working as designed but not working as desired; repairing the same system would imply something like eliminating the work-arounds that people have developed to get around the failings of the system.