VA-REST Log-in System using mysql (wordpress website)

Basically, im developing a game that we can register from a wordpress website. And to log-in we should use wordpress websites database . I’m using php 7.3 and wordpress 5.8.3 .

how can I use varest plugin to:

“check if there is a user with these informations > if there is → get user data > log-in the user” ?

Does your wordpress website have REST API enabled? If so, then check the endpoints for logins/registrations and you should be able to send JSON data to those endpoints using the VAREST plugin from UE4. VAREST only works with web servers that implement the REST API. So if you do not have it implemented on your web server, then you will have to do it manually. But I think wordpress gives you that functionality. If you are using, check this out:
REST API Handbook | WordPress Developer Resources