I have been building with my 6yo, and he has the traditional giant bin where all the sets of Creator pack of assorted bricks, plus the sets from Ninjago and Spider Man and Minecraft and various cars and so on all get disassembled after about 3 days for him to dig through.
More recently, I also got out my old boxes of Knex, which I'd put away a decade ago by sorting the parts by color into about a dozen quart-size ziploc bags (there are far fewer variants of Knex than of Lego, even ignoring the myriad custom tiles and stickers). He was THRILLED to have them all sorted, and for more than a month now - probably a dozen sessions of use - has put them back in the right bags. It's like pulling teeth to get him to keep his art supplies organized, those can just be piled in a heap, it can take two hours for him to do his laundry, typical playtime with friends is an explosion of toys from bins, but it is critical that the Knex go in the right bag even during use.
After seeing that, I got a compartmentalized organizer that used to hold fishing tackle, dumped all the tackle in the big tackle box, and washed it, and he has been keeping "the good Legos" in that. If you're curious, it turns out that the good Lego are the wheels, propellers, blocks with pins to connect to those wheels and propellers, the Technik couplers, the Ninjago transparent flames, especially the Ninjago spring-loaded bolt gun thing, and the Minecraft character heads Not the character bodies, not the Iron Man head, not even the ones that look like him and Mom and Dad, just Minecraft heads - everything else can go in the bin. I think he made a good selection, builds just seem to go together faster when those parts are available. In particular he used to need help on occasion to find the right wheels - everything seems to need wheels - and now he has them.
God I hope he's more organized and tidy than me.