Does your phone actually need antivirus?

Phones are built very differently from the computers antivirus was invented for. Apps run in separate compartments, and installing software is more controlled. That changes what a security app on a phone can do, and what it's for.

iPhone

iOS keeps every app in its own sandbox, and apps can't look inside each other. That means no app, security or otherwise, can scan your other apps the way desktop antivirus scans files. Security apps on iPhone focus on what they can reach: blocking dangerous websites and phishing links, filtering suspicious messages, a VPN for untrusted networks, and checks on your device settings. Useful, but different from "antivirus".

Android

Android is more open. You can install apps from outside the Play Store, and apps have more room to work across the system. Google Play Protect, built into most Android phones, scans apps from the store and from elsewhere. A third-party security app adds its own scanning, especially for apps installed from outside the store, plus protection against malicious links. The more you sideload, the more a second layer is worth.

The threats that actually hit phones

Phishing by text, email and messaging apps. By far the most common, and it works on any phone.
Malicious or overly greedy apps, often clones of popular apps, or ones asking for permissions they don't need.
Out-of-date software. Security fixes only help once they're installed.
Losing the phone, with no screen lock or remote wipe set up.

The free basics come first. Automatic updates, a strong screen lock, apps only from official stores, and "Find My" turned on. A security app adds to these; it doesn't replace them.

When a security app is worth it

If you're on Android and install apps from outside the Play Store. If you often use public Wi-Fi and want a VPN in the same place. If you, or someone you set phones up for, tends to tap links in messages. If you'd like one subscription to cover phone, tablet and computer together rather than three separate tools.

A five-minute phone check

Install any pending system update, and turn on automatic updates.
Delete apps you haven't opened in months.
Review permissions for location, microphone, camera and contacts, and remove what doesn't make sense.
Make sure your screen lock is a PIN of six digits or more, or a passcode, plus biometrics.
Turn on Find My iPhone or Find My Device, and check remote wipe works.