To access folders other than the Inbox via POP, log in with the special username username+foldername@domain.tld. For example, if you have a folder called "Hiking", you can access this folder via POP by logging in using the username username+Hiking@domain.tld.
To access subfolders, change the username to username+parentfolder.subfolder@domain.tld. For example, if you have a subfolder named "Hike" under the parent folder "Hiking", you need to enter the username as username+Hiking.Hike@domain.tld.
Please note: Folder names are case-sensitive, so the username must be capitalized to match the folder. Additionally, this method of accessing folders will not work if the folder name contains special characters.
If you have multiple folders which you would like to see on your mail client, it is easier to configure the account as IMAP instead of POP. The advantage of IMAP access is that the mailbox structure is maintained exactly as on the server. You can find the IMAP setup instructions on the Settings → Migration screen.