You cannot hide from advertisers if you use a smartphone with apps. App developers who put ads within their app control the apps behavior completely and hence they can fingerprint your device and track you very well without using IP addresses. And within browsers, they can fingerprint you through many javascript features of the browser. Hiding your source IP does very little for your privacy.
Almost all traffic (apps and websites) are encrypted via TLS (https, for example). So, even if you are on an insecure network, unless your OS's TLS certificate store is compromised, your communications are encrypted and protected against snooping from that insecure network.
Also, even on open wifi networks, today, it is very unlikely that the wifi is running without at least WPA2 encryption. Most modern airports run secure wifi. (But they also monitor all traffic metadata for illegal activities).
So, using a VPN as an exit node is just privacy theatre. VPN exit nodes in faraway countries are useful for bypassing content censorship in your own country, but it works only if the content streaming service cooperates with you.
Remember, all ISPs are heavily regulated by governments and can be asked to mirror specific customer's traffic for analysis. I would be very surprised if they don't proactively do it for all VPN operator nodes by default.