How to redirect a menu item to the anchor?
Modules are how you extend Drupal's functionality; there's very little point saying you want to achieve something without a module, as if one exists, you're just going to end up copy/pasting its code into your own module/theme anyway to get the same effect.

Drupal core has no functionality to link to an anchor, so you need to install a module which does, or copy the code from one to your own.

Whichever of those methods you decide to use, Special Menu Items has the functionality you require (you can either install
and use it as normal, or pick the relevant bits of code out into your own module/theme).