So in European Portuguese the "s" is pronounced like a Z if it's between two vowels. But also in estamos the e sound isn't pronounced, so it sounds more like shtamos.
So when pronouncing Nós estamos, is it pronounced like
It's not necessarily true that "the e sound isn't pronounced". It depends on factors such as the speed of speech and formality. It can be pronounced but reduced to /i/ or /ɨ/, in which case a speaker might say /nɔ.ziʃ.tɐ.muʃ/, if speaking slowly and formally, for example.
Otherwise, a speaker would say /nɔʃ.tɐ.muʃ/. If there's a pause between nós and estamos, which there usually wouldn't be, then it'd be /nɔʃ.ʃtɐ.muʃ/.
/s‿ʃ/ is possible, but less frequent, so /nɔs‿ʃtɐ.muʃ/ is possible too. The issue you're concerned about is technically called "consonant sandhi", and there's a brief summary of how it works on Wikipedia.
It all depends where you are from in Brazil and which words.
I am from the northeast and my whole family is from the northeast, so I pronounce S differently.
But in your examples, the second one "noz eshtamos" remind me a lot people in Rio de Janeiro, because they tend to pronounce S as SH.
But I would say with the "SSSS" sound, (like a snake) not with Z sound or SH sound.
In this vídeo you can have an idea, but even her is pronouncing words ending with S as SH, which is not necessarily wrong or right.
The S has different pronunciations, depending on the word containing it. For example, in casa (house) has an accent for Z, but in castelo has a listenable sound of S. The sound depends from the intonation of the word. I recommend you to learn the International phonetic alphabet (IPA) to better understand how words are pronounced.
