Do I need an antivirus on my phone? If I do, which one is best? (android)

And what about getting rid of ads?

As far as need, everything is sort of relative. The more Internet surfing and click bait links to chase the more you need anti virus software.

For anti-virus see if whatever antivirus software you are using on your computer has a mobile app. Typically paid for antivirus software will also include a mobile app.

I would look at a VPN, especially if you connect to a lot of free open Wi-Fi hotspots.

Same thing usually applies with vpns. Paying for a VPN usually includes a mobile app and the ability to install on multiple devices.

Add blocking may already be a feature built into your antivirus or VPN. Each one's a bit different and features may depend on free versus paid.

Not really. If it includes anti-phish protection...then maybe if you are susceptible to that. Use Brave as your browser, helps with ads and some extra security/privacy protection. VPN is a good idea if you use public wifi.

Even on Windows PCs....most attacks revolve around phishing and account compromise these days. Even ransomware, the origin of a bad attack is often someone got phished/social engineering. Awareness, strong password practices, and MFA will keep you out of 99.9% of the trouble you can control.