What's Hotspot Texturing?
Hotspot texturing is a trim texturing technique made popular by Valve Software. Known uses of this technique date back as far as 2014, Matthew Trevelyan Johns states: this method is by no means new, rather it's quite an old technique. I first used a very similar method and automatic trimming tool, way back on 'The Lego Movie Videogame' (2014). 
It also shares some similarities with the "Ultimate Trim" from Sunset Overdrive  and traditional trim sheets. But what makes hotspot textures unique is that they have a variety of rectangles and squares baked with clean bevelled edges. Allowing you to have nice bevel edges for a variety of surface areas.
As stated in Valve's documentation it is "a method to quickly apply textures to geometry by matching faces to predefined rectangles."  In Half Life: Alyx it allowed for quick texturing of bulky objects or environment geometry using predefined rectangles and square texture patches. In Hammer 2 there is a UV tool that can automatically unwrap these hotspot textures to geometry, making level design much faster to iterate on.
What does a Hotspot Texture look like?
An example of a hotspot texture from Half Life: Alyx:
As shown above you can see a variety of different sized rectangles and squares. Each one has a clean bevel on each side. From here the hotspot texture can be textured to represent whatever surface is required.
This texturing technique was used and taken further with the Last of Us Part 1. Matthew Trevelyan Johns  shows that this technique can be used to texture a variety of props. This also improved performance as less textures are needed to be used per unique prop. Johns States: "The newer process allows us to achieve a higher resolution result using re-usable trim shaders, improving the quality whilst reducing texture memory costs" 
I would highly recommend checking out the ArtStation post here as it goes into detail how this technique was used at Naughty Dog.
How can I use Hotspot Textures?
When the Half Life: Alyx workshop tools appeared, hotspot textures caught the attention of many alike. From this tools that help with this technique soon came about in many popular 3D tools. Here is a list of tools that can assist with hotspot texturing.
Mesh Tool https://www.unrealengine.com/marketplace/en-US/product/mesh-tool (Paid)
(If anyone knows of any tools for Max or Maya or any other software leave a comment down below!)
For my experiments and testing with hotspot textures I've been using Dream UV for Blender.
How do I layout my Hotspot Texture?
The first thing to figure out was the layout for each section. From looking at Half Life: Alyx the variety of hotspot texture layouts can vary greatly.
As stated by Dave Watts "There is no perfect layout for every use case, as it depends what scale of surface you're using it on" 
This is clearly the case in Half Life: Alyx as wall hotspot textures tend to get fewer sections versus a rusty metal hotspot texture that seems to be designed for more intricate meshes.
For the first test I decided to try a wall layout of sections as defined by Valve.
The next challenge would be figuring out the bevel width for each trim section. According to Dave Watts "the important thing is bevelling every piece with 45-degree angles, and making that bevel the right size for your texture" 
I would highly recommend checking out Dave's breakdown here.
With this we can then inset and pull up each section making sure that it's at a 45-degree angle. After this adding a bevel modifier helps crisp up the edges.
Quick test baking out the normal map. We can now use Dream UV to unwrap this to our geometry.
And here it is applied to some geometry:
The bevels really help add depth to the wall sections. Notice that the underlying geometry is all hard edges. This is important to ensure the bevels are rendered correctly.
So where can I get Hotspot Textures?
Great question. Besides creating them yourself there aren't any purchasable hotspot textures* (as far as I know (2024) comment below if you know of any!)
So, to solve this issue we are now offering a few hotspot textures for sale.
For those interested in making their own hotspot textures but don't want to setup the sections themselves, we are now offering a base mesh to act as a starting template to create your own hotspot textures:
For full PBR hotspot texturing materials look no further than these products offered here:
Get started with out first materials, Concrete and Plaster:
Make your own! Get this high poly model to help you get started with baking out the necessary textures.
Want to blend between them? Look no further than this 5 pack of mask textures.
Let us know if you use these! All feedback would be great as this will prompt us to potentially create more in the future.