Yes its like Dropbox but self hosted. It also provides a number of other features but for the sake of simplicity well only think about the file sync.

WebDAV is just a way to talk to nextcloud over http to send and receive data. Keepass2Android supports WebDAV, thus I use that to manage the syncing of my database.

The effort to self host is…well its just subjective. I’ve been doing this for a long time and my profession directly applies to these things so for myself I don’t consider it much of an effort. If you are starting from zero though there will definitely be a lot to learn.

I don’t use a bought NAS but you can certainly do so. I have some experience with the Synology devices and they seem OK but I don’t know what its like running different third party services like nextcloud on it so you’d want to read up.

I host everything on systems I’ve built. My home server is just some old desktop hardware that I put into an supermicro tower chassis. My main server is colocated at a data center which I also built from eBay parts and put into a 2u 12 bay supermicro chassis.

So you don’t have to buy some nas to get started, just need some PC hardware.