Steam Game Server
Browse and join listed game servers.

Introduction

Available in the Steamworks Complete asset.
The Steam Game Server concept is often confused when you first start looking into Steam and its related tools, interfaces and services.
Game Servers (Steamworks Documentation)
Please click the link shown above or again here and read Steam's documentaiton on the subject.
In the most basic since when we (or anyone) says "Steam Game Server" also shortened as SGS what they mean is a game server that initializes the Steam API with a Steam Game Server configuration. by doing so the SGS can be browsed for through tools like Steam's Game Server Browser and it can leverage key features of the Steam API directly.

What can it do?

Networking

Heathen does not provide networking (HLAPI) tools.
You will need to use a tool like Mirror, MLAPI or FishNetworking, etc. as your networking high level interface.
Heathen provides access to Steam APIs and does so in a standardized way such that each of the afore mentioned provider's Steam Transports work with Heathen's Steamworks assets
To make that clear
There is zero customization needed from Heathen to enable you to work with Steam networking. Simply use Mirror, MLAPI, FishNetworking or any other networking highlevel API that has a Steamworks.NET transport.
The main advantage to configuring and initializing the Steam API on a game server is the ability to use Steam Networking interfaces with it. Valve provides several networking interfaces but two of them in particular are relevant for Unity game developers
Steam Networking
This is not limited to P2P games despite what some of the API names suggest
Yes Steam has noted that they would like to deprecate this interface in favor of Steam Networking Sockets. No it has not been removed from the API yet and there is no publicized date for such.
As a point in note Steam API still has old authentication logic from more than a decade ago in it e.g. they are not likely to ever break your game by removing API features.
and
Steam Networking Sockets

Authentication

The Steam Authentication tools must be used with SGS if you want Steam Game Server Browser to properly list user counts. The Authenticaiton documentation can provide more details.

Stats and Achievements Security

Stats and Achievements can be a great way to develop your game's community and can even serve as a progression system, however, its extremally easy for user's to set stats and achievements them selves without your game at all.
Stats and Achievements can be configured such that they can only be set from Game Servers (GS) or Official Game Servers (Offical GS). This provides a layer of security and makes it feasible to use Stats and Achievements for game play impacting or community impacting features.
It is not to say you must use SGS for Stats and Achievements only that you must accept that every client based feature will be exploited by your users.

Game Server Browser

Put simply server discovery. Steam Game Servers can be configured to list on Steam's Game Server Browser. This can then be used in or out of game to discover game sessions to play or spectate. See the Game Server Browser Manager document for more information

Hosting

No
Put simply Valve (Steam) doesn't publicly provide hosting ... yet ...
They have teased that they are looking into it but they are a private company they tease lots of things that never come to pass and sometimes things just come out of the blue and are wonderful.
In short there is no facility at current available to you that we are aware where you can host a game server on Valve's servers.
There are great tools available to you though a few are linked below. We do not sponsor or endorse any of them, this is purely to get you started on your own research

Configuration

To configure a Steam Game Server browser simply set the desired settings in your Steam Settings object and build a server build. In particular take note of the Game Server settings.

Hosting

At this time Valve (they build Steam) does not offer hosting you will have to provide for that your self.

Player Hosted

This means that you ship your server build and let player's host them, them selves. It was a popular method in the past and many games still allow for it especially if you want to foster a LAN community.

Bespoke

Abuse of the term but what is meant here is a hosting service provider. These are usually used to let your player's host servers. Minecraft, Ark, Conan Exiles, this is a popular model with survival games.
Look for providers like GPortal to help with this.

Backend Service Provider

The new traditional method, using a backend service provider, most if not all of these offer hosting of some sort along with other services.
Look for providers like PlayFab, GameSparks and Multiplay

Proprietary

There are some options that offer great advantages but that are proprietary systems so moving off of them is not really possible at least not without a lot of work. We would only recommend you consider these if your game REALLY needs the advantage they offer and your happy to marry your game to there offerings.
Look at providers like Improbable

DIY

Do it your self, not recommended for ... well most people and even large organizations. It is exceedingly difficult to host a live service game of any kind without a lot of money and experience and 24hr a day staff.
That said it is an option and you can offset some of the cost by using basic hosting services like Azure, AWS, etc.