Yes building tools from scratch is a great way to learn. In my carrier a rewrote bunch of front-end frameworks, but then instead of rewriting everything from scratch I decided to simplify existing structures.

Once I tried to create my own front-end framework from scratch. I created signal library, mounting/unmouting, context etc but never able to write v-dom diffing or using existing vdom libraries.

Then I decided just implement my API on top of Preact. Preact did vdom-diffing for me and mounting/unmounting which was enough for my need.

The final project looked nothing like Preact, it even had it's own context system, data loading, signals, setup + render and context which just hooks itself into Preact Context but taught me a lot about how a front-end framework work.