The Apple TV box gets its input over the home network. There isn’t really an option to store content directly on the device. The storage is for apps, system software, settings, and cache files only.
Your collection of video files can be accessed through the Apple TV box, if they are on a networked computer (or NAS). You can then launch a special app to access the files there remotely.
Apple offers Home Sharing for this. The personal video files would then be loaded into the Apple TV app (or iTunes app) Library on a networked Mac computer. You can then activate Home Sharing on that computer. The shared Library is then accessible on other compatible home network devices, such as the Apple TV box through the Computers app. Files need to be in a compatible format for this: mov/mp4/m4v with H.264/H.265 video and AAC or (E)AC3 audio.
Third party apps like Plex, Infuse, VLC, etc. offer a similar service with e.g. more file format support and NAS support. These do not rely on the Apple TV app (or iTunes app) Library. Some of these apps need to have a component installed on the serving computer or NAS. Some of these apps require additional payments.
If this sounds like a hassle, then you can try AirPlay or AirPlay Screen Mirroring. Possibly you like that better. That is however operated (start, pause, stop) on the Mac, not on the Apple TV box.
