/ə/, a different vowel sound from /e/, is never written with "é"
In the context of the French language, the notation "/e/" standardly represents a specific vowel sound. This vowel sound does not occur in the first syllables of retourner, premier, demander. Those words all have a different vowel sound, transcribed "/ə/", in the first syllable: they are pronounced /ʁətuʁne/, /pʁəmje/, and /dəmɑ̃de/. It would be incorrect either to pronounce them with the sound /e/ in the first syllable or to spell them with "é" in the first syllable.
The letter "e" without an accent often represents the sound /ə/. The sound /ə/ is never written "é" (I don't know whether there might be one or two exceptions, but this is true as a general rule).
Some French words have /ə/ written "e", and other words have /e/ written "é": there isn't a simple rule to predict which of these two possible pronunciation/spelling combinations occurs in any particular word. You just need to memorize either the spelling, or the pronunciation, or both.
The vowel /e/ and the spelling "é" don't occur in syllables that end in consonants
The spelling "é", which almost always represents the sound /e/, is usually only possible in a syllable that ends in a vowel sound (called in linguistics an "open syllable").
In a syllable that ends in a consonant sound (called in linguistics a "closed syllable"), the spelling is standardly "e", with no accent mark, and the pronunciation is for most speakers the slightly different sound /ɛ/. (I said "most" because I'm not sure about all accents. I know that in some other positions, the use of the sound /e/ versus /ɛ/ differs between accents.)
Phonetically, the sounds /e/ and /ɛ/ differ in "vowel height", which refers to the height of the tongue: it is raised a bit higher for /e/ than it is for /ɛ/. Because a raised tongue tends to "close off" the mouth more than a lowered tongue, a different term for high and low vowels is "close vowels" and "open vowels". So /e/ is often called "close e", versus /ɛ/ which is "open e". Don't get confused by the different meanings of "open" when talking about syllables and vowel sounds.
For historical reasons, vowels before double consonant letters (which are pronounced the same as single consonant letters in modern French) or before consonant clusters starting with "s" are treated like vowels in closed syllables for the purposes of spelling.
In expliquer, the first syllable ends in a consonant sound, so it is written with "e" without an accent and the word is pronounced /ɛksplike/, with /ɛ/.
In détester, the vowel in the second syllable is followed by "st", a consonant cluster starting with "s", so it is written with "e" without an accent and the word is pronounced /detɛste/, with /ɛ/.
For comparison, the vowels in the first syllable of détester and the first and second syllables of répéter occur in open syllables and are only followed by a single consonant letter in the spelling, so the sound /e/ spelled "é" is possible in those positions. (The sound /ə/ spelled "e" would also be possible in the same positions: there is a word spelled relever that is pronounced /ʁələve/, or with elision of the middle schwa, /ʁəlve/.)