This has 2 key things:

* "well designed": What was the objectives and ideas...

* "codebases": How well that was implemented

They are a lot of lofty claims saying how this or that is "fast, secure, etc" but don't end like that in the actual implementation.

But most of the time, that could be seen in the "design claims" already! Good design is not just full of adjectives and nice sounding goals, but the concrete considerations, what was the trade-offs, false-starts, and reasons behind the decisions.

You can see some examples reading about the design of Erlang, early pascal, most RDBMS, etc.

So, you first mid/long term goal is to learn to distinguish what good design actual look like.

then, in relation with codebases then to be kinda easier: It actually follow the design?

A good example is the 'std' library of Rust. It has a lot of lofty claims about security and such things that could sound alarms, but then you dive in the code of it and see is there A LOT of care about it, and a lot of docs comments discussing this stuff and then the code match.

P.D: The "std" or equivalent of the lang is one of the most important codebases you need to learn and study, and the MAJOR way to judge how truly good is it.