Employers look at a resume first, then they look at the cover letter. They typically look at the resume first to make sure you have the desired skills and experience before taking the time to read your cover letter. This is especially true in fields that require specific hard skills, such as IT and engineering.
However, if you’re applying for a role that requires writing skills, such as editor, journalist, or copywriter, employers may read your cover letter first to ensure that your writing abilities are at the level they expect.