Once Oni Gallery is installed, you will have:
The dashboard pages provide:
Onmi Gallery is unique in decoupling the selection of images from how they are displayed. If you have already used or read about Universal Content Puller, Omni Gallery does the same kind of thing for image galleries and sliders.
With this kind of flexibility, you are unlikely to use it all on one site. What you have is choice on how you configure and use image galleries and sliders and flexibility to easily change or reconfigure without having to go through all the faff of using different blocks.
At this point, its best to pick a small fileset, like the elemental default images we have added to a fileset in our examples on this site. Then save and publish and see what it looks like.
Having checked that works, adjust the display widget settings to suit your site design, save and iterate as necessary. Then once everything is looking good, go back to Image Selection and adjust it to work with your full set of images.
If you need to present a similar gallery on other pages, you can use the settings tab to Export the settings and Import them on a new Omni Gallery block, or even import them to the default settings for all Omni Gallery blocks.
You can even use this Export/Import of JSON settings to copy gallery designs between sites. Though take care to make sure filesets and folders are adjusted as per (4) above.