How to make modern mobile versions of sites?

Question

Mobile development

Hello,
I noticed that sites like VK, or Toaster, work differently.
It is mobile versions of sites.
Does anyone know what technologies are used in the development?
Works very fast and without errors.
If there are special front-end frameworks for mobile versions?

Answer 1

I noticed that sites like VK, or Toaster, work differently.

I do not agree with Toaster, the usual adaptive layout.
You can even tritely try to shrink the browser window from the computer and it will look the same as from the phone.
And on VK, yes, a separate mobile version - https://m.vk.com/
Do a redirect as you like, checking the devices from which you entered.
If the site is in php, a good solution is mobiledetect.net

Answer 2

Check the user-agent, check the permission of the user's browser and return the corresponding page