I haven't watched any myself, but I found some just by searching "godot 3d navmesh gridmap" on duckduckgo.
https://godotengine.org/asset-library/asset/350
⬆ This is an asset you'll need an older version of Godot 3 to use, but the setup should be close enough to give you an idea of how to do it.
If you are still having trouble finding exactly what you're looking for, you can also try asking on the Godot Discord. A lot more users pass through there than here, so you might have better luck.
