Why does Mail on macOS Monterey remove emails after 30 days?

Hello,

My wife uses Mac Mail primarily for Gmail and also uses iCloud - both of her inboxes keep mail for only 30 days.

I use Mac Mail primarily for iCloud and also use it for Gmail - both inboxes keep mail pretty much indefinitely.

I have looked at settings on her two Macs for both Mail.app and on gmail.com, but I don't see anything that mentions removing or keeping emails for a fixed amount of time. I've seen other forums discuss this problem, and the answer always points to removing deleted emails, but these are emails that are in the inbox and were not marked for deletion.

When my wife looks at her inbox in https://mail.google.com/, it corresponds with her inbox on her Mac - only 30 days of emails.

What is happening here?

In the old days of Eudora, you could specify to remove emails off of the server after a certain length of time, but I don't think that is what is happening.

She does occasionally access her gmail from her iPhone. Is there something in the iPhone Mail app that removes emails after 30 days? On her iPhone, she does not use Gmail.app, but Mail.app.

She and I have the latest released version of macOS and iOS.

Thanks for any help.

David