questions in Your undertale AUs

so i have a question for anyone who makes undertale fan games....how do you do it? i've been thinking of making a undertale fangame myself but idk how (i asked this a while back but game engines like game maker costed money so i need a alternative)

I was thinking about this for a while. How come the Undertale AU names have an exclamation point in them? For example, Swap! Undyne. Are you supposed to yell, or...it is just emphasizing the fact that the character is from an AU?