In response to your second question and video there - to start with, if you are making a C++ project it is best to choose the C++ templates tab when creating it rather than the Blueprint templates.
Moving UE4 files from “Content” by copy-paste can have issues so it is generally best to use the Migrate feature via the UE4 Editor instead (although you have to be in the project they are from to be able to migrate them to another project’s content folder). Using source control would avoid this issue if you have something break that you cannot resolve as you can revert to an older build that worked and migrate files from that build.
The reason for your project working through creating a new project is because you did not copy over the C++ files that were giving you errors in the original project.
Simply creating a new project for a different version will not resolve version difference issues, if there are any. Also, especially so with larger projects, a lot of settings specific to a project are done via the Editor which can become time-consuming to ensure you transfer settings and default values over 1:1 between projects without missing something. If neither of those bother you then creating a new project instead is completely fine, just understand that in most cases the errors on switching version is due to your own errors so transferring over any files that include errors will result in the same result you were having with the original project.