Why do my emails disappear?

What mail app are you using? Who is your email provider? How long are they there before they go away? Are they gone or moved to trash/deleted?

I don’t think your emails are deleted. The iPhone simply stops displaying them after a certain length of time. I would use the app specific to your e-mail (outlook app if you have an outlook email address) and you should be able to view all your emails.

The last answer makes sense. I have a hotmail, outlook address and they are the ones that go away. The gmail ones stay. Why didn’t this happen with my old iPhone 7?? Is there a setting I can change?

Most phones have an email setting similar to ' days of email to sync'. This means these are the only emails brought to your phone. They are still online, just not brought to the phone, change the sync time if you wish.
Not sure about iphones but for Microsoft accounts, if you delete them using the phone they are deleted online.

MS accounts, if they are past the sync days go to https://outlook.live.com/mail to see old emails.

Settings, Passwords & Accounts, select the Account, find Mail Days to Sync and adjust accordingly.