Hi Jean-Claude,
Please Control-click on the videos and choose the option to download them. This way you can view them even with low bandwidth. These are not video tutorials, but they will give you some ideas about how to use Observatory.
Also please read the Getting Started section of the documentation, as well as the Glossary (especially the Observatory section).
An Observatory Library is a special document that hold references to your images and organizes them in folders/albums/smart albums. It's like an Apple Photos library, but where with Apple Photos you only have one, with Observatory you can have many libraries. That allows you to organize your images in many different ways.
An Album in such library is a container for your images as well as stacks of images. It could contain for example all images you obtain in one night (I typically use a YYYYMMDD naming scheme for those), but you could also create them for each object.
The real fun starts if your images are plate solved (Observatory can do that for you if they are not). Plate solved images will be automatically tagged by Observatory, and besides it displaying catalog overlays for them (e.g. galaxies, planetary nebulae, variable stars, confirmed exoplanets, etc), it will give you much more power to search for them (e.g. show all images with a globular cluster in them).
