My pictures on my computer are scatter among many different drives, folders and subfolders. Is it possible to use the search features on all of these simultaneously? Or do the pictures have to be in just one folder?
No, your pictures do not all need to be in one folder for PM to find and index them, but you do have to tell PM exactly where to search.
IOW, assuming you are using the Advanced view, in the left hand column, under the "Folder"option, you should see all your connected drives listed. Clicking on any of the drives will show you the folders on them; clicking, in turn, on each folder that contains images will allow PM to index them.
Clearly, this will take time!
On my computer I don't use the default "Pictures" folder at all. All my photos are in one main directory called "Images", with sub-folders for each set of related images. My RAW images are kept on a separate drive, in a dedicated folder, again with sub-folders. Indexing any of my images is thus a matter of clicking on either the "Images" or the "RAW" folder. PM then creates its database.
So, to (sort of) fully answer your question: to have PM index images etc. which are scattered among many different drives, folders and subfolders is possible, but initially at least, rather time-consuming for you, the user since you must tell PM where to search.