How to pronounce "stupid", "YouTube" and words like this?

In dictionaries like Cambridge or Oxford it pronounced like "stjupid". But I hear how some people pronounce it like "stchupid". Is it different dialect or what's the reason?

I assume that with stchupid you mean to represent a pronunciation starting with something like [stʃ] in the International phonetic alphabet (the "s" sound of "say" followed by the "ch" sound of "chew").

In British English, words that dictionaries show as having syllable-initial consonant clusters /tj/ and /dj/ are often pronounced with phonetic affricates [tʃ] or [dʒ]. This has been discussed elsewhere on this site; see Nardog's answer here. Likewise, /stj/ can sound like [stʃ].

In British-English that would be "stew-pid"; in American-English that would be "stoo-pid"