This collection of online "courses" is an excellent resource at all stages of learning. There are courses for beginners, intermediates, and experts, and always something new to learn. Courses come with sample data, videos, and step-by-step instructions. Courses can range from minutes to tens of hours in length, and can be in many formats: Tutorials, videos, web courses, instructor-led, teaching courses, etc.
The University of Ottawa has a institutional license that allows students, faculty, and staff to have access to all these courses for free (exception: instructor-led). Browse the catalog of courses here: https://www.esri.com/training/catalog/search/.
Here are recommended ArcGIS courses to get started: