SD-WAN vs Site to Site VPN's

I'm looking in to a couple of options to get SD WAN set up at my company to allow more remote working.

I currently use a Unifi USG3 at home and know it has site to site VPN capabilities. How does this differ from SD WAN? Is this something Unifi can support?

Background:

We have 4 locations where people are currently based. As things stand, data sharing is quite archaic. If we want to share files we email them across, no shared locations / network drives.

We are looking to maintain these 4 locations, however also want to allow people the opportunity to work from home as and when they wish, yet still have access to everything as if they are in the office.

We also have sales reps out on the road who could do with access to centrally stored information, as well as our CRM platform.

We can do most of this over Office 365, however things like shared storage for non Microsoft items would be nice, as well as a secure connection for our CRM and payment processing platform.

Thanks