They’re not using “free” images.
As a designer, you can pay stock agencies to acquire usage rights to their images. In the case of Adobe Stock you can pay about $10 for a ‘standard license’ to use one of the furniture cut outs. Your client hires you to make a flyer for them, you pay $10 for a standard license, then deliver a finished flyer to them, and they can print up to 500,000 flyers. If they want more than that you can pay for another standard license.
Say you want to make a template like the ones above, then you need an ‘extended license’. You pay Adobe Stock $80, and then you have the rights to place that same furniture cut out into a flyer, then sell that flyer on Adobe Stock… assuming it passes their inspection and they accept it into their collection. It’s important to note that you can’t turnaround and sell that image by itself, on Adobe or anywhere, but you can incorporate it into templates you’ve designed and products for resale.
Keep in mind that if you sell your newly created furniture template on Adobe Stock they really don’t pay you much. Most of their sales are on subscription, which will net you around $0.20 per download. At best, you might get around $3 per sale. It may take you awhile to earn back the $80 you spent on the license, and even more if Adobe’s search algorithm decides to bury you on page 5000 of a search return. Buyers might never see your template.
One workaround that some people use is to not include the high res licensed image in the template. The buyer purchases your template, opens up the layered PSD file, sees the furniture layer with a low res watermarked placeholder image on a layer that contains a link to the web page where they can secure the rights to the image. They license and download, then place it on the layer so that the clipping path you created can be applied to it. Istock used to forbid that kind of usage. Each stock site may have it’s own policy.
Make sure you understand the licensing before doing this. It’s a mess, and it varies by site, and they’ve all got different standards for what constitutes an extended license, and some like Getty are quite litigious. And on top of that, photographers and illustrators can opt out of making their work available through extended licenses. Keep documentation of your licenses.