Authorization in web config

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ASP.NET

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<authorization>
<deny users="*" />
</authorization>

I understand the above block of code will deny all users ..

My Question is : What will this do ?

<authorization>
<deny users="?" />
</authorization>

Will it check if the logged in user is authorized ? If yes HOW and WHERE should i write a code..

I have seen people writing

<authorization>
<allow users="Admin" />
</authorization>
But i cant manually write this if i have multiple users whom i want to give access how would i do that ?

My understanding says i can write a Function which hits a database and check if UserID =1 has access to PAGEID=1 then let him continue further else land him on NOACCESS.aspx.

But then what does the above blocks in web config do ?

What I have tried:

i have written function to fetch user access from database and handling on page load