As an example of this:

Here's a hinge mechanism: https://files.catbox.moe/06g9cg.png

Here's the very first sketch in the timeline, I've just clicked on this to edit it, there was no undoing required: https://files.catbox.moe/xzr3un.png

Here it is overlaid on the model, which just requires a single mouse click to toggle: https://files.catbox.moe/339fvg.png

Here it is after I changed a bunch of numbers: https://files.catbox.moe/pvykrx.png

Here's what it looks like when I press the button to show the final model again: https://files.catbox.moe/6bjc0c.png

Just like that the entire model has changed based on the initial sketch. I didn't have to go edit any other features here.

As a bonus here's the assembly being moved by dragging the mouse after those changes, showing that this might not be the most well-designed hinge after those changes: https://files.catbox.moe/eztkja.mp4

Also here's the timeline I mentioned, you can see how I've clicked on a feature and it shows me exactly what it did, this also works when clicking on part of the model: https://files.catbox.moe/f0bb2k.png

Well, like I said: From open tools I had available b123d was the one that relatively quickly (within a weekend from picking up the tool) allowed me to mock up something to the point where I became confident enough to keep going.

I tried to do that with FreeCAD without too much success. I assume someone proficient with it could probably crank out the whole thing within an afternoon, but not me.

I'm sure professional tools are many times more productive, but you need to spend thousands of dollars and presumably months if not years of training to become proficient in them.

And admittedly that's more of a weird hangup of mine but to me it also sort-of misses the point to use a proprietary tool for building an open source project. No one without the tool could open the source files and make modifications.