I’ve been using paid ChatGPT version from the very beginning. However, recently I found it to be degrading rapidly. I did a comparison of 4 different AI models today - GPT4, 4o, Perplexity, and Claude. I found the Perplexity’s free version to provide the best outputs. I started reading about it trying to understand what model it uses and the paid version says you can choose from Gpt4o, Claude-3, Sonar Large, etc.
My question is - what model does standard and Pro versions use? I liked its free version and it says that it’s the Standard Perplexity AI model. Is its proprietary software? So if I switch to the Pro version, what model would I be using? Will it be a combination of the ones listed above and I’d have to choose or will it be one advanced version of own Perplexity AI model?
I consider myself still somewhat a beginner in this field and these may be obvious type questions for most on this forum, so please bare with me. 
Thank you!