Look at list https://www.inthecloud247.com/free-online-courses-for-microsoft-365-and-azure/ or https://www.microsoft.com/en-us/learning/mct-readiness.aspx It contains free available MOOC course for modern Microsoft technologies like Azure, widely understood Workplace services (including current Exchange 2019 exam MS-200 an MS-201). All of those courses are hosted on https://openedx.microsoft.com/courses/ - and require to login using Microsoft Account (Office 365 credentials doen’t work).

Personally I prefeer that format, which is mix of text, talking heads, training and publicly available labs.

For Exchange postmasters I strongly recommend all 5 Exchange related courses:

MS-200: Planning and Configuring a Messaging Platform

• MS-200.1 Understanding the Modern Messaging Infrastructure
• MS-200.2 Managing Client Access and Mail Flow
• MS-200.3 Managing High Availability and Disaster Recovery

MS-201: Implementing a Hybrid and Secure Messaging Platform

• MS201.1 Defining a Hybrid Messaging Strategy
• MS201.2 Managing Message Security, Hygiene and Compliance