Does APKLinker.com host paid or pirated apps?

APKLinker.com does not support piracy so it does not host paid or pirated apps.

What are these terms added with APK title – dpi, nodpi, arm, arm64, x86?

Android runs on a variety of devices that offer different CPU architectures and/or screen sizes.
Some applications are optimized for different CPU architectures and/or Screen DPI (Dots Per Inch).
CPU architectures are arm (armeabi, armeabi-v7a), arm64 (arm64, arm64-v8a), x86, x86_64, mips, mips64 etc.
Screen DPIs are 120, 160, …, 640 etc

Will I be able to update the app from the Play Store, If I install an APK from APKLinker.com?

Sideloading an APK file from a site like APKLinker.com will not affect updates you get from the play store.
As soon as the Play Store has a newer version of the app, you can update from there also.