What should I prefer, One Page Checkout or Multipage checkout and why?
Hey social1,
This is a Bubble forum – dedicated to share and get information about developing apps on the Bubble platform. I see in your other post you’re working with Magento, not Bubble… 
Anyway, as someone who worked with Magento with a long time, I can try answering your questions:
What should I prefer, One Page Checkout or Multipage checkout and why?
Opinions on this from UX experts are still varying on this. One page checkouts have everything on one page, but can be quite overwhelming. Multipage checkouts are less overwhelming, but there are more steps where to user can drop out… A lot of popular eCommerce platforms seem to be switching to Multipage checkouts (like Shopify) – and they probably did their research on it…
An eCommerce store or Multivendor Marketplace, which one is better? How can create a marketplace for my Magento store?
Which one is better is totally up to you. Depends on what you’re selling, who you’re selling to, etc… You probably can create a marketplace with Magento, but as far as I know this is not a native functionality and you’ll need a plugin for this.
Now you’re already on the Bubble forum… Try giving Bubble a try! You can create online stores and Marketplaces with Bubble as well! I can really recommend trying this platform out! 
Cheers!
In my opinion, both one-page checkout and multi-page checkout are not optimized because your potential customers need to fill many fields in many steps/pages. The best solution for checkout process is one step checkout.
If you are using Magento, I suggest you should use Magento 2 one step checkout extension. It merges all checkout steps and pages into 1 page and shows all fields you need to fill. As a research, this extension helps to increase the conversion rate by 20%.
Neither of the extension is the right thing in the current situation as both requires some time which might result in loosing the attention of customer and abandon cart.
However, I think there is a solution to that you can use Magento 2 OneStepCheckout Extension as it minimizes the whole purchase process and speed up the tasks by reducing it to just one click.