Omeka S is a web publication system for universities, galleries, libraries, archives, and museums. It consists of a network of independently curated sites sharing a collaboratively built pool of items, media, and their metadata.
Omeka Classic was designed for a single site around a general theme, while Omeka S is a multisite system. Because of this, Omeka S is recommended for institutions managing multiple independent projects, which can share a resource pool.

Just like Omeka Classic, Omeka S is an open-source project, so the application is free to install and use as long as you have the necessary means.

Omeka S has a different inner structure, but items, including metadata and media, can be migrated from Omeka Classic by using a module.

No, there is no limitation.

The most popular metadata set used in Omeka is Dublin Core. It is a small set of vocabulary terms that can be used to describe digital resources (video, images, web pages, etc.), as well as physical resources such as books or CDs, and objects like artworks.
Two forms of Dublin Core exist: Simple Dublin Core and Qualified Dublin Core. Simple Dublin Core expresses elements as attribute-value pairs using just the 15 metadata elements from the Dublin Core Metadata Element Set. Qualified Dublin Core increases the specificity of metadata by adding information about encoding schemes, enumerated lists of values, or other processing clues.

Learning about Omeka can improve your employability or benefit your current institution from its possibilities. The platform is very useful for libraries, archives, museums etc. as a manner of organizing and publishing contents about collections and research. Many institutions can benefit from this software, and some are already appreciating this skill in hiring processes.

The official documentation is at https://omeka.org/s/docs/user-manual/.

Item sets are just an aggregation of items. It is similar to Omeka Classic collections. In Omeka S, items may belong to any number of sets, and an item set can have any number of items.

A user with the role of the global admin is someone who controls everything and is typically the person who created the Installation.
A site admin is an administrator of only certain sites within an Omeka S installation, and they cannot modify global settings.

A set of predefined metadata fields to be used for certain kinds of items, in order to guide Item creation and interpretation of Properties.

Videos from Youtube and other platforms can be added as item media or they can be directly inserted in the site (Omeka S) or exhibit (Classic) pages.