Which SEO tool to start with
For a site you own, start with Google Search Console and Bing Webmaster Tools. Their reports show how those search engines see your pages, including search performance and indexing. SimilarWeb serves a different purpose: its traffic figures are estimates for market research, not a substitute for your own search data.
Use PageSpeed Insights when you are investigating page performance. SEO audit tools can surface items to review, but a checklist score alone does not tell you whether a page answers a visitor's question or deserves to rank.