Search engines use many different ways to find new pages. Most commonly their web crawlers follow (external as well as internal) hyperlinks.
While a typical informational website links to all available pages in its site-wide navigation (so web crawlers can reach all pages by following internal links), other websites don’t necessarily link to all their pages (maybe because you can only reach them via forms, or because it doesn’t make sense for them to provide all links, etc.).
To allow discovery/crawling of new pages of these sites, too, they can provide a site map. This is essentially just a page linking to all existing pages, but often with structured metadata that can help search engines.
So just make sure that all your pages are linked somehow. Either via "natural" internal links on your site, or by providing a sitemap (ideally following the sitemaps.org protocol), or both.
For questions about SEO advice (which is off-topic here on SO), see our sister site https://webmasters.stackexchange.com/.
