Merging Folders

You could first create one new folder. Then select all of the folders to be 'combined'. Then drag one of them into the new folder. All that have been selected with be transferred. The command for selecting all is to press the Command + A keys.

If I understand correctly, you want to put the contents of all of those folders, and not the folders themselves, into a single folder.

Do these folders contain only files, or do they contain also some subfolders?

If they have just files, I can offer an easy solution.

0) First, put all those folders inside a folder, if they are not already.

1) Open a Finder window to this folder - let's call this folder "A".

2) Press Command-F to start a search; select "A" instead of "This Mac".

3) Hold down Option key; the + button for adding a search criterion changes to "..." Click.

Change the criteria to be like this:

Your search will return all the files inside all the folders inside "A".

4) Press Command-A to Select All

5) Drag these files to the desired folder