Lead Designer will really do more "general design" across the board, in terms of "how the game plays."
It's a bit out of my element and I'm sure there's some collaboration (Hey we want to add this art... cool I can do some fun stuff with this art!).
But it's about overseeing the gameplay from things like puzzles, creating encounters, and so forth.
There's a variety of aspects that they will oversee, but like "Level Design" is one part, and they are the ones that take the level art (the geometry with all the placeables) and start to craft encounters and so forth.
Tech Designers will set up how puzzles and stuff work, do the scripting for the scenes. THere's a lot of overlap, but yeah.
ANd of course, it's all still collaborative. Writers will have plots, and the level designers will start placing those conversations and whatnot int he scene, while art guys go "yeah this looks good" and so forth
But yeah, "design" is sort of the "not programming and not art" aspect of content creation. If that makes any sense?