Unity Learn is a free website by Unity technologies to teach game development with Unity. It's best if you want to become Unity certified.
Free Unity tutorials arranged in a way to understand game development. You will be ready to create your own game with Unity after these tutorials.
Udemy is a huge pool of course for everything. It also contains many free coursed to get started with the game engine of your choice.
There are multiple YouTube channels that can help you learn game development. It's best for people who have some basic knowledge on game development.
Unreal has a complete tutorial series to help new game developers learn Unreal engine from scratch. Unreal has a steep learning curve than Unity.
Godot is an open source game engine and has become as powerful as Unity and Unreal. Their documentation is good even for new developers.
They have tutorial for all game engine and are easy to follow. Their tutorial are made by expert writers and are made keeping new developers in mind.
Free Code camp
Free code camp has multiple game development courses for new developers. You can use it to learn the game engine of your choice.
If you wish to learn the basics of coding before going into game development, then Simplilearn is the best place to start.