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As soon as the Play Store has a newer version of the app, you can update from there also.

How do you make sure that all uploaded APKs are real?

All APK files are checked before being posted. The digital signatures must match with other applications or releases from the same developer.

What is .apks file?

It is a zip archive file with ".apks" extension. Many apps are migrating to the split APKs format that cannot be easily installed just like regular ".apk" file. ".apks" file contains multiple APK files including Base APK and Split APKs. You can use SAI (Split APKs Installer) that can install such apps using standard Android API, using root access or using Shizuku.