May I use online tutorials that are available free to the public, as
texts for courses I teach withoutinfringing on copyrights?

There is no correct answer here. You would need to ask the copyright
holder for permission to use the text.

The content may be open access, in which case you can use the content.
However it may have tight copyright controls because it is a paid for
tutorial, in which case you would be taking business away from the
copyright holder.

Your only course of action is to ask the copyright holder for
permission to use their content. Many educational providers are happy
to do this for small groups.

All the best