How do I make the same reduction as the aluminum part in Sketchup Web?
It’s hard to tell exactly what you’ve got there but I would draw a cross section with the exterior features and use Follow Me around a circle to create what I think you are showing.
Somthing like this:
As a follow up to Dave’s excellent comment, you could make the profile the same way but as an outer and subtract that from your original shape using solid tools. But I now notice you are using free web so that’s not available.
Solid tools are not available in the WEB version but the Intersection is possible.
Did you model it with Follow Me as I showed? If so, why not add the chamfer in the profile before running Follow Me?
How are you using the model once you have it created? That can inform how you should model it.