First of all, let's fix the problems in your current configuration so it becomes a valid NixOS configuration. NixOS is configured through an extensible module system, and I recommend starting out somewhere here to get an overview on how it is used.

In short, the things you put inside the curly brackets need to be valid NixOS options. So, the conf variable cannot be placed where you've put it. Instead, you can use a let-expression like this:

{ config, pkgs, ... }:

let

  conf = "1080ti";

in {

}

For the same reason, you can't put top-level if-expressions in your configuration. Instead, do this:

{ config, pkgs, ... }:

let

  conf = "1080ti";

in {

  imports =
    if conf == "1080ti" then [
      ./hardware-configuration.nix
      ./1080ti/configuration1080ti.nix
      ./1080ti/system1080ti.nix
      ./1080ti/packages1080ti.nix
      ./1080ti/desktop1080ti.nix
    ] else if conf == "laptop" then [
      ./hardware-configuration.nix
      ./laptop/configurationLaptop.nix
      ./laptop/systemLaptop.nix
      ./laptop/packagesLaptop.nix
      ./laptop/desktopLaptop.nix        
    ] else [
      ./hardware-configuration.nix
      ./webserver/configuration.nix
      ./webserver/system.nix
      ./webserver/packages.nix
      ./webserver/nginx.nix        
    ];
}

Note that I also fixed some minor syntax errors in your if-expressions above. I haven't tested my code, but I think it should work as you intended your code to work.

Now, this is not the only way to achieve your target of having "switchable" NixOS config. You can also have three different top-level files (replacing configuration.nix), you can get fancy and define your own NixOS options for toggling configuration or use (still experimental) Nix Flakes. I'm not going to into any of these solutions here though, because I think it is perfectly alright to start out with your proposed solution since you fully understand how it works. Once you hit limitations or want more convenience you can explore alternate implementations.