Learn Turkish: Bilim insanları her şeyi bilir mi?

Because the sentence is asking about a general, timeless fact (do they know?), not an action happening right at this moment. The verb stem bil- gets the aorist suffix -ir, followed by the separate question particle mi.

Why is the verb singular (bilir mi) when the subject (bilim insanları) is plural?

In Turkish, if a plural subject is explicitly stated in the sentence, the verb often stays singular. Because scientists are human, you could technically say bilirler mi, but leaving the verb singular avoids repetition and sounds very natural.

Why is it bilim insanları and not just bilim insanlar?

This is a noun compound. When two nouns join to make a new concept, the second noun must take a compound suffix. Here, bilim (science) and insanlar (humans) are linked with the suffix -ı to create the word for scientists.

Why does her şey take the -i suffix to become her şeyi?

The verb bil- (to know) needs a direct object. In Turkish, words like her şey (everything) or herkes (everyone) are treated as specific, known objects. Therefore, they must take the specific direct object (accusative) suffix, which is -i in this case.