How are you using 'really'?
1. With the exclamatory addressee-directed pragmatic usage [Merriam-Webster; adjusted]:
b1 —used to emphasize a [following] assertion [especially a corrective, usually
showing exasperation]
...
- Really, you're being ridiculous!
Hence (the comma to show a discontinuity after 'No', optional in the 'Oh really' case, as 'Oh really!' is a fixed phrase):
- "No, really! That is how it actually works."
...................... or ....................
- With one of the sentence-modifying (traditionally, sentence adverbial) usages Merriam-Webster; adjusted:
a2: in reality : [a deep analysis shows the truth to be]
a3: truly, unquestionably [I kid you not] [a veridical pragmatic usage]
He looks like an idiot, he talks like an idiot, but don't let this fool you. He really is an idiot. [Groucho Marx]
36 Jesus himself stood in the midst of them, and said, ‘Peace be with you.’ 37 They were terrified and alarmed, and thought they were seeing a ghost. 38 ‘Why are you so disturbed?’ he said.... 39 Look at my hands and feet; it really is me, myself. Touch me and see! Ghosts don’t have flesh and bones like you can see I have.’ [Luke 24; NTE; Bible Gateway]
Either of these two usages need the 'really' to be attached to the matrix sentence;
- "No. Really, that is how it actually works."
Usage a3 would be distinguished from a2 in speech by emphasising really is.