Screaming Frog - With the help of the crawler tool, you can identify keywords that are already covered by the site, since the output contains all the meta titles of the whole site. This approach can also be used for keyword extraction from a competitor's website in order to get the seed terms.
Specialized SEO tools - With tools like SEMrush or Ahrefs, you can find important information, such as the search volume, ranking potential, keyword seasonality, content gap report and related queries. All this data will be useful later on in the prioritisation stage, so that we dont optimize for terms that no one is searching for.
Other data sources - Other sources of relevant keyword idea might also include your own industry knowledge, related searches in SERP, “people also search” section, PPC campaigns or an internal search box directly on the web.