TH assimilation in English

Hi, I'd like to clear this matter. In English, when we got a TH sound ( like the, this, that, than, though...) I found out that we've got some kind of "assimilation". For example, if before the TH sound we got a N, the TH disappears ("in the" becomes "inna", "in this" becomes "innis"...), Later I discovered that this also happens when there's a L ("fail the" becomes "failla"...) and sometimes when there's a Z ("is this" becomes "izzis"...). So what about the case of "What is the", does it become " what iz ze"?. If it does then wouldnt it sound the same as " what is a"? Thanks a lot