"The Tectonic authors have to admit that it’s been a long, long time since they’ve needed a LaTeX tutorial, so they’re not sure what the best resources are. Consider making a suggestion in a new GitHub issue if you’ve got one!"
I've got one:
https://www.overleaf.com/learn/latex/Learn_LaTeX_in_30_minutes
Note: I used Overleaf in the past and have no affiliation with them or their employees.
I've got one:
https://www.overleaf.com/learn/latex/Learn_LaTeX_in_30_minutes
Note: I used Overleaf in the past and have no affiliation with them or their employees.