When I create a new post, can I choose an uploaded file from a specific folder?

Yes, you can choose the folder in the dropdown list of the filter box beside the search box.

How can I bulk select multiple files and put them into a folder?

You can use the bulk select Button in media grid, bulk select files you want and just drag them into a folder.

Do all of my uploaded files remain the same after I first install this plugin?

Yes, they do.

If I create a folder or category, does this override the default WordPress path “wp-content/uploads”?

No, it does not. The file default path still remains.

When I move an image to another folder, does this action affect the image appearance on my frontend?

No, it does not. The image is still displayed correctly on the site frontend.