I have watched several videos on youtube about Dutch pronunciation and read several posts on Duolingo and elsewhere, but I am still confused by the pronunciation of the 's' sound. Everything I have read and watched seems to suggest that the s sound is essentially similar to that of English. But the Dutch voice on Duolingo (the computer version, not sure if the mobile app is different) definitely sounds like it is pronouncing at least some of the 's's as "sh" (as it is in certain German words, like "Stadt"). Moreover, Forvo seems to suggest that at least some native speakers pronounce "s" as "sh" in words like "rijst" (https://forvo.com/word/rijst/#nl) or "sap" (https://forvo.com/word/sap/#nl). Am I missing something here? Are the phonetic conditions under which "sh" is an allophone of /s/? Or is there dialectical variation in the pronunciation of 's'?
Here's what Wikipedia says: "In the Netherlands, /s/ and /z/ may have only mid-to-low pitched friction [s̻], and for many Netherlandic speakers, they are retracted. In Belgium, they are more similar to English /s, z/."
Basically, it means that the /s/ sounds is between an English s and an English sh. As our ears are not trained to make a distinction between [s], [s̻] & [ʃ], it can get very confusing for us.
I listened to the Forvo pronunciations. I don't hear any SH sounds. I think for both Rijst and Sap the person named Nobellius came closest to how I would pronounce those words. With Dutch, you have to keep in mind that there a lot of regional variations in pronunciation. Standard Dutch (what they're teaching here on Duolingo) was actually formed from all the regional Dutch languages, but the different pronunciations still persist. It's oftentimes easy to tell from a Dutch person's speech, which province he or she comes from, much like you could probably distinguish a Bostonian from an Alabaman by their speech. You're right that the Dutch S is pronounced like the English S, unless it's a word that starts with SH, but there aren't many if any of those in Dutch.
I've had a lot of problems with that, too (you can see my comment here) and didn't get any convincing answer. Only when I encountered the same problem in Greek, I found better explanations, especially thanks to the Wikipedia article on Greek phonology, and stopped thinking about it. See my thoughts on that here.
Or is there dialectical variation in the pronunciation of 's'?
Plenty, plenty of dialects and plenty of variation on about every sound, grammar point and word you may encounter in the Netherlands. For as far as I can hear, the Dutch 's' is a Voiceless alveolar sibilant. (may not mean that much to you.) Here's the audio: https://en.wikipedia.org/wiki/File:Voiceless_alveolar_sibilant.ogg