If I understand correctly, your current folder structure is based on year and now you want to create a monthly bifurcation under the respective years and organize the images in those folders?
If my understanding is correct, would mind confirming if you have created those sub-folders already or if you would be doing that?
You can add a new sub-folder to your folder structure by selecting the folder under which you wish to create the new folder and select the option to Add sub-folder.
If you have the images that you wish to add to the new sub-folder already selected then check the option to include images and give the folder the desired name. You can also drag the images over to the new sub-folder later after it is created.
You can also check the information mentioned in the link below.
In the Library>Grid View select the year folder in the left-side Folder panel. Select the photos that you want to move. Right-click on the year folder and select "Create Folder Inside ..." and give the new folder a name. Click the "Include selected photos" box and "Create".