Good question. I'm still learning Perplexity, so take all this with a grain of salt. So yeah, the free version of Perplexity is their own tuned (customized) version of 3.5. If you get Perplexity Pro, you continue to have access to that model. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.