Not only does the data, but all the apps & the device itself still belong to the original Apple ID.
Apple's policy isn't that anything belongs to the human holding the device, it belongs to the owner of the ID.
Without that ID/password & no way to obtain it or contact the previous owner, yet with access to the phone itself & presumably its passcode, the only option will be to transfer what data you can off it, then throw the phone away.
There are methods to gain full access to the device itself if you can prove ownership - see How can I bypass Activation Lock? - but none of these will gain you access to the existing data, merely allow you to re-purpose the phone under new credentials.
EaseUS web site has How to Transfer Contacts from iPhone to Android in 4 Easy Ways. They are, of course, trying to persuade you in the end to consider their own software, though they show a method to transfer contact data to Google, which will then give you access to the contacts as .vcf files.
I'm not certain that you can really get right through any of those methods without at some point needing the Apple ID & password.
If you hit that at any point, then your method will be reduced to typing it out by hand into another computer.
For WhatsApp, I don't know of any method that doesn't involve paid [& often quite dodgily-marketed] software, which I've never tried [& am never likely to.] You'd have to do some further research... & caveat emptor, read reviews on sites not connected to any of the software makers.