Online courses can also be used as teaching material. There are tons of so called “MOOC” platforms and here’s only a selection that we find useful.
Here’s a platform that allows you to search for all kinds of materials and media that are protected by the creative commons license.
Open Educational Resources are teaching, learning or research materials that are in the public domain or released with an intellectual property license that allows for free use, adaptation, and distribution.
A selection of free sources.