Questions about fixed timestep game loop

I am at the part where I am trying to implement a fixed timestep game loop

I have read articles like:
https://gafferongames.com/post/fix_your_timestep/
https://dewitters.com/dewitters-gameloop/

And I understand the basics as follows:
1. Calculate the current frame's delta time
2. Add the delta time to a “update time” counter
3. While you have enough “update time” left in the counter update your game logic and minus the target timestep / FPS amount
4. Calculate a interpolation value and pass it to the render method

My confusion sets in and everything kind of falls apart with …