Like Wordle In A Day ?
Or by word search, do you mean the kind where you are given random letters in rows and columns and you have to circle rows, columns, and diagonals that include words?
For that kind of puzzle, you build it by working from lists of words (Google is your friend), and linking them together on the empty grid so they cross at common letters, then filling in the blanks with random letters.
P.S. I had the wrong idea of the Juan_Antonio solution by its title (Crossword). It actually is a word search, on examination. Around here, crossword means numbered squares and external hint lists covering every horizontal and vertical word.