Here the abstract noun "likeness" means that what Ezekiel saw looked like "an expansive dome." The word can be translated with a verbal phrase. Alternate translation: "what looked like an expansive dome" (See: [[rc://en/ta/man/translate/figs-abstractnouns]])
"and the expansive dome was spread out above the heads of the creatures" or "and the expansive dome took up a lot of space over the heads of the creatures"