What do you want to know? Basically it's just another set of GNSS satellites that devices can use for positioning, if their chipset supports it. Customers benefit greatly from this.

Do we benefit already? I have no idea which positioning system is in effect. E.g. I am in Russia - Have I just used GPS, Galilleo, Glonass or combination of all 3 of them (iPhone 8).

Your iPhone is indifferent to the type of the system and likely uses several of them at once.

Some (many? all? I have no idea) consumer chipsets with multiple GNSS support can get position from multiple constellations, but cannot combine the signals into one mixed constellation result of better quality than the best single-constellation result they get. In most cases what you see is the GPS result, except when you are close to one of the poles, which are uniquely (afaik) served by Glonass.

On android there are countless apps that show you the satellites currently received, I guess there are similar apps for iphones as well.

> I don't know if and how they are used in 2018

Even 100 USD android phones use GPS and glonass and have done so for a while.

It decreases the time needed for a fix for example.