Topaz Labs made stand-alone apps for various image processing tasks - these could also be called as plug-ins from Photoshop. These were apps like Clarity, Detail, InFocus, DeNoise, etc.
A couple of years ago or so, Topaz came out with an all-in-one image editing environment called "Topaz Studio." It is a stand-alone app that can also be called as a Photoshop plug-in. It has several "adjustments" you can apply to an image - think of these as adjustment layers in Photoshop. Some of these adjustments are called by the names that used to be apps (like clarity, detail, etc.) because they perform the same task as the former stand-alone app did.
The way that Topaz Studio is made available to users is that you can download and use the basic application for free. This gives you the environment plus basic adjustments. You can acquire more adjustments by paying for them - most of the time these adjustments are denoted as "Pro" adjustments. Most of these also have trials. Studio Pro is a group of several advanced adjustments that can be purchased a la carte or together as a single package price that gives you a discount over a la carte pricing. Further, Topaz Studio has GPU acceleration and multiprocessor enhancements that make it faster on more modern machines.
If you previously purchased the stand-alone Classic version of an adjustment that is available in its new Studio form, then you can activate that adjustment in Studio for free (at least that is what I recall doing for the Classic apps that I own).
Topaz Studio is also how Topaz rolls out new tools, like the new AI Clear, which is AI-powered noise reduction and sharpening.
Here's where you can download everything:
Hope this helps.