The site has two kinds of tools.
Free public tools
The SEO tools and developer tools pages are open to everyone. They check one page, one domain or one piece of text at a time. Visitors who are not signed in have a daily limit; signed-in users can keep going, and tools that collect search data use credits.
Account features
Projects, site audits of a whole website, daily rank tracking, saved keyword lists, domain overview, backlinks and the AI tools are in your account, under SEO. These keep history, so you can see change over time.
A good rule: use the free tools for a quick look, and add a project for anything you want to follow every day.