"it" and "this" are not interchangeable.
"this" introduces something new to a conversation by metaphorically pointing at it.
"it" is a personal pronoun and refers back to something you had already spoken about previously.
"This is Duolingo" = Dies ist Duolingo. or Das ist Duolingo.; a bit more colloquially also Das hier ist Duolingo.
It sounds like that to you because you are not used to hearing spoken German. It's perfectly normal.
Every language has its own set of sounds, some of which are the same as English, some, not. When we are children, our brains "tune" to what we hear around us, and we become very good at picking out the sounds of our native tongue, but other sounds, not so much. Just keep listening to as much German as you can find, and your brain will begin to "get it".