1) The administrator can transfer files, securely using SSL, to and from a remote location by using free open client source software such as WinSCP or FileZilla.
2) SSH allows an administrator to manage the server from afar using a command line. Most likely, few people will manage their advanced home server in that way.
First, download FileZilla client software or WinSCP. Read what the hosting web site says about your new software and look over the documentation. There’s other SSH software available. Use it if you want.
Second, acquire a URL name using a Dynamic DNS provider (DDNS) if you don’t already have a web presence. You will not need to install an SSL root certificate. Your SSH client will ask you if you trust the one your SSH client finds. Since you are essentially talking to yourself in this instance, I would assume the transfer is safe.
WebDAV is a more user friendly tool for secure file transfers to and from a QNAP NAS device. Users other than the administrator may use WebDAV and more granular control can be had over what may be accessed. But SSH is a traditional feature, so instructions about it are included.
QNAP SSH Setup
Sign into QNAP administration and turn on SSH. (By the way, SFTP has nothing to do with FTP or FTPS. They just have similar names.)
Open port 22 on your router.
Start you SSH client.
Admire your work.