App was not getting started due to <system.web> tag

Question

I am developing a desktop app with .NET Core Which consume data from Rest APIs and save that in database. In past I used .NET framework to develop such app and I use App.config to get connectionStrings to connect database and to get other values. There was also other tag like startup, supportedRuntime, system.web, runtime etc.

Now I am using same file to get same data in .Net Core and It is working fine.

Then I do some research and found appsettings.json is recommend for .NET Core.

Here My questions are:

1. Does startup tag make any impact for .NET Core or is it use less as I am using

   <startup><supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5" /></startup>

Which is wrong as I developed this in .NET Core not in ".NETFramework,Version=v4.5".

2. App was not getting started due to <system.web> tag, so I am commented this, Now App is running successfully. Will making this tag impact other functionality of App.

<membership defaultProvider="ClientAuthenticationMembershipProvider">
        <providers>
            <add name="ClientAuthenticationMembershipProvider" type="System.Web.ClientServices.Providers.ClientFormsAuthenticationMembershipProvider, System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" serviceUri="" />
        </providers>
    </membership>
    <roleManager defaultProvider="ClientRoleProvider" enabled="true">
        <providers>
            <add name="ClientRoleProvider" type="System.Web.ClientServices.Providers.ClientRoleProvider, System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" serviceUri="" cacheTimeout="86400" />
        </providers>
    </roleManager>
</system.web>```
    

    3. What should be best practice in this scenario to use configuration.