There is no hard and fast rule as to whether CV or cover letters are read first (CV are also commonly called a resume). Some employers and hiring managers claim to read the cover letter first (because so many applicants don’t bother to include one), and others say that they read the CV first to ensure a candidate has the essential qualifications and experience.
Typically, it would be safe to assume that for positions that require specific skills, experience or qualifications, recruiters and hiring managers would look at the CV first to make sure you have them before taking the time to read your cover letter. This is especially true in fields that require hard skills, such as IT, accounting, architecture, or engineering.
However, it would stand to reason that employers and hiring managers might read your cover letter first to ensure that your writing skills are at the level they require in more creative fields such as journalism, copywriting, and customer service.