I think I understand yor situation a bit better. The advice above, that is to Pkg.add/dev all the folders applies to standalone packages/projects.
For your filestructure, which I think is a single package, you would instead use submodules.
You can simply include("Module1.jl") from Main.jl. Then to access Module1 from Main’s scope, you would do using .Module1. From Module1 you would do using ..Main to access names defined in Main. The dots are there to define the scope of the imports.
See Modules · The Julia Language and How to arrange a package with submodules and multiple files? - #6 by Gu_Junhua
I think that should answer your questions. I believe the only difference from Python is that you need to include the file (prior to doing using/import), since using and import don’t look things up by filename. This does mean you need to make sure you don’t include the same file multiple times.