On the web: ArcGIS Pro latest version online help (use Search box at upper right or explore tabs). You can also get to this from the Windows Start Menu, under ArcGIS...ArcGIS Pro...ArcGIS pro Online Help.
Within ArcGIS: Press the F1 key, click the question mark ? in the upper right, or click Help under the Project tab: you'll go to the online help in a browser window.
The Esri Training Sitehas many different sorts of training videos and other resources. the search interface lets you limit to ArcGIS pro and the format of help that you want. Under E-Learning, the Training Seminars are typically 1-hour free webinars on more general topics. The Tutorials are also free resources, typically shorter and on more specific topics, and some of the shorter Videos are also free. There are also multi-week MOOCS, some of which may relate to ArcGIS Pro.
Esri Training Site: Licensed
The Esri Training Sitealso has some resources with a notice "Requires Maintenance." These simply require "Esri Access" to be enabled through your ArcGIS Online account. Email email@example.com ask enabling Esri Access to access these training materials. Such licensed resources include, under the E-Learning format group, the Web Courses and the longer Videos.