There have been a few bugs in the past years that let you take over a phone without user interaction. There was one where you only need to receive an SMS (it was invisible even) and your phone is infected. Another one was a vulnerability in wifi calling and voice over lte.

A phone is not a passive device that only gets something when you request it. You take also it with you to public places, use it in open wifi networks and you get calls. All that while being used for security critical stuff like 2FA, banking and payment.

You shouldn't use a phone without current security updates for much more than calling. It is a time bomb. If you want to educate yourself further you should look at "zero click vulnerabilities".