You access a plug-in from a host (such as PS, Lightroom, PS Elements). Some of Topaz can be used stand-alone. I use Topaz and Nik Collection with PS Elements and work with layers.

As for your comment, I look at the Topaz tutorials and wonder if the original 'photograph' is still in there...., methinks you're not going to get the full benefit of additional software (although I see you did ask about watercolor effects not long ago) PS Elements is all you need for basic editing and more.

Playtime with Topaz Simplify and PS Elements:

Apologies if I didn't read the earlier comments close enough, but for 'layers' and the LR users with Topaz plugins, another method is to purchase the Topaz PhotoFXLab. This tool gives you a single interface between Lightroom and all the Topaz plugins. Pass the image from LR as a 16-bit TIFF in colorspace - ProPhoto RGB. Once inside the PhotoFXLab, you can build layers as you step through the Topaz editors, if you use multiple plug ins. For example, I may take an image from LR to Topaz. My first step is to duplicate that layer. Then, I might apply Denoize. I duplicate the results of the Denoise layer and then apply HDR processing in Adjust. I duplicate that layer and then do a B&W conversion. All these duplicate layers are not technically required, but you might compare the results by hiding version layers and see something you want to redo / do differently. You can restart with one of those incremental layers rather than having to cancel out of the FXlab and start over.