Ages ago I was taught Russian at the behest of my then-employer. Gaining the skill to switch accents to meet our immediate needs was very important, so we spent a lot of class time on it.
Running two words together, which is what's happening in the second version of each example, is the mark of a fluent (it means "flowing", and might be the word you want as a general descriptor) speaker. All native speakers do it, and it makes some languages very hard to parse for the listening learner. One word flows into the next seamlessly, audible pauses only occurring where they must.
There is also a social-class difference, however. People of higher social classes generally speak more slowly and distinctly, running their words together more subtly. Vowels are "rounder" (fewer schwas), consonants more defined (fewer glottal stops).
So "want to", "orange juice", and "is she" are more likely to be clear when spoken by someone of a higher social class, or someone trying to emulate a person of that class (that's the other meaning of "assimilation": social assimilation.
If you hear "wanna", "oranjoos", and "ishy", you may be listening to a native speaker from the working class, someone who, in most countries, gets little or no decent education (because someone whose life is going to be spent digging tatties or hauling away rubbish to the tip isn't worth spending money on, if you're a certain kind of government spender).