Recently the Content Type Hub in SharePoint Online turned into the Content Type Gallery and this is more that just a rename of a feature.
The Content Type Gallery in SharePoint Online allows you to easily manage and customize content types across multiple sites. It enables administrators to centrally define, manage, and apply content type settings across multiple sites. This makes it easier for organizations to create consistent experiences when working with documents and other data stored in SharePoint Online.
Using the Content Type Gallery helps ensure that all users have access to a common set of options when creating or editing documents within their site collections. It also simplifies the process of managing existing content types by allowing admins quickly modify existing ones or create new custom ones without having to go through each individual site collection one at a time. Additionally, admins can use the gallery as an easy way to view which content types are being used on each site collection by checking out its usage report feature.
To add a content type to a list or library
Go to the document library in which you want to update the content type with a document template. On the Ribbon, click the Library tab, and then click Library Settings. Under Content Types, click the name of the content type that you want to change.