Certain pages get crawled more often than others. As a website owner, you can tell Google how often your content changes, which they will use as a clue for how often to recrawl your site.

If you're really big (like for example Reddit), you actually lose that control in Google dashboard -- they fully control the crawl rate. From experience, I can tell you that they are crawling large sites, like Reddit, continuously. Even 7 years ago, when I last worked there, they were crawling Reddit so much that we had to set up a separate server infrastructure just to respond to Google's requests, because their access patterns were so different from every other user.