How are mobile application interfaces designed?

Question

Mobile development

Never interested in mobile development, only web (php+js+frontend frameworks like Vue). I am a complete zero in mobile development, but it is curious how colleagues in the neighboring workshop do it.
Here on the web there is html + css and JS (at least for working with the DOM, for example), they study it in high school in computer science classes. And everyone knows what html and css are. The nuances and work with individual browsers are learned in practice.
And what about mobile applications?
For example, in a mobile application, there is a main header with a menu and other buttons. There are all kinds of animations on click on some element, transitions, transforms, opacity, etc.
What is all this written in mobile development? Do they use css and/or html syntax to show this header, and this footer, and is it styled like this? And here are divs with inline-block, and if you click on it, then such and such javascript is applied.
Do Mobile Development Use CSS3 Animations? Properties like transition: width easy-in-out, transform etc.?
Or is it all written using pure Java (for Android), and Swift for iOS, using a different syntax for each language? In other words, to make the same interface for Android and the same mobile interface for iOS, two specialists are needed, Java'ist and Swift'ist, respectively? What about different screen sizes? There are media queries in css and this is convenient, if the screen is such and such, then we move the button here, and if it’s different, then we move it there. And what about this in mobile applications?
Nubian question, sorry, not banned in Google, but did not find anything sensible. I know that now you can write mob. applications using frontend frameworks like Angular and React, that is, everything written in the world of understandable languages ​​​​(html, css, js) is compiled in Java (or Swift).
But these frameworks appeared much later than the first mobile applications appeared, and it is not entirely clear whether the use of modern frontend frameworks when writing mobile applications is a pattern or still an anti-pattern.

Answer 1

Object sizes (view, layout) are not absolute (in pixels), but relative (march_constrait, wrap_content). It is possible in pixels, but in principle. Relativity helps with screen rotation.