Quickest way to find competitors keywords?

Google Ads Keyword Planner does a version of this for free. Pick "start with a website", paste one of their URLs and it suggests keywords based on that page. You need an ads account but you don't have to run any ads.

Drop their domain into the organic keywords report of whatever SEO tool you have access to and sort by position, the stuff they rank 4-20 for is where you can actually compete. No tool budget? Look at the titles of their top pages and run those topics through autocomplete yourself.