Lessons often might need models, textures, and other assets to represent your ideas to their fullest. Here are some asset repositories and useful tools that are free of charge, open-source, or royalty-free!

3D Models
Turbosquid 
CgTrader
SketchFab
TinkerCAD
Smithsonian 3D
GrabCAD
Thingiverse
Unity Asset Store

2D Assets
Pixabay
Unsplash
Wikimedia Commons

Sound Effects/Music
Freesound
Youtube Studio

Modeling and Creation
GIMP
Audacity
Blender
Musescore
Meshlab
Krita

Recording and Sharing
OBS
ShareX

Storage for Arena Files
Dropbox

Conversion (Arena supported GLB)
glTF to GLB
Aspose: FBX to GLB
Any to GLB