How do most ASP.NET developers memorize every libraries and classes? Do you have any tips?

I think, in the .Net environment we have good APIs in general. Lots of usages can be inferred just by the naming.

Example:
"I want to serialize json" (newtonsoft)
-> JsonSerializer

"I want to setup this mock to return a specific value" (moq)
-> ...Setup(<selector>). Returns(<value>)

I've never had to memorize anything. Documentation is your friend. The rest is learned from repetition.

@iamavalos thank you for reply! So whenever I need something I can refer to the official Microsoft docs? What about the case I don’t understand what I need? Do I need to study guidelines Microsoft provide from scratch?

@fermar7 in your example, yes everything looks simple
And instinctive. But how do I know which setup and configuration I need when I develop something?

@creativeJay regarding this I would also, as the others already said, stick to official docs or even "unofficial" ones like blog articles for example.
I think very important for this is to express what you want to do in a short and condense phrase to be able to use google or search indices in documentations to find what you need.

@fermar7 thank you for the advice :)

Tip: you bind the "quick fix" command to a hot key.

Type the class name that you can remember, for example "convert" to convert base64, etc

Visual studio now says error, cursor on the error and press the quickfix hot key. It will suggest and add the full name for it.

Quickfix can also add null check, implement props, etc.. for you

@iamavalos seconded

I think in .NET Intellisense from Visual Studio is also helping a lot

Thank you all of you! All of your advices and reply made me feel I have people who support and be with me :)