Mail app on my iPhone not showing all mails.

Created a whole account on this just to reply and give you an answer since it took me so long to figure it out too.

On your iPhone go into Settings > Mail > Accounts > Click on your mail account > “Mail Days to Sync” > Click “No Limit”

this is what made them all finally appear for me.

Scrolling down in the iOS Mail app continuously populates the content because IMAP (Internet Message Access Protocol) is the default protocol used by the iOS Mail app. This protocol allows the app to dynamically load and display email messages as you scroll, providing a seamless and efficient way to access your email without the need to manually fetch each message individually.

Unlike POP3 (another email protocol), IMAP keeps copies of your emails on the server, so even if you scroll down and seemingly "use up" the emails on your phone, the Mail app can retrieve more from the server as needed. This makes it seem like your inbox keeps refilling.

In simpler terms, the Mail app on iPhones and iPads shows your emails from a server, and since it keeps copies there, you can keep scrolling down and see more emails as you go.