Alliteration is the recurring sound of a consonant. That is, any sound except a vowel. Sometimes several consonants play together, weaving in and out. Alliteration comes naturally to all of us, including poets. Without even thinking about it we use it in nicknames, find it in comics and nursery rhymes. From my childhood I remember Desperate Dan, Korky the Cat, Lucy Locket. Advertisers exploit it. Poets sing it. Children chant it. Poets using Old English over a thousand years ago, and even earlier in the much older British language (Welsh), relished alliteration.
Poets don’t think about using alliteration, assonance and all those tricks. It comes naturally. I’ve just checked some of my poems for alliteration, and find plenty there. ‘The Field Mouse’ begins with ‘s’ sounds in the first line:
‘Summer, and the long grass is a snare drum.’
and ends with three lines full of ‘b’ sounds, a few ‘l’s, and again that sighing ‘s’:
‘their bones brittle as mouse ribs, the air
stammering with gunfire, my neighbour turned
stranger, wounding my land with stones.’