Rock on bottom or just soil?

Pure soil all the way to the bottom, just squash it down around the holes. The plants' roots will go all the way to the bottom, they need soil there, not rocks.

i add 2-3 cm of clay ball under the soil

just soil

Dryer sheets.

Used to do this. anything large enough will work - perlite, leca balls, et al. It's probably more risk averse and some small but measurable benefit in root zone, but since stopping i haven't noticed an issues with my roots and still shaving some time off vege cycle without it. The roots come out lily white at end with some staining possible. I've yet to have any diseased root zone in nearly 5 years so far -- did it the first year, but not since. Not sure how commonly it occurs. Might need a decade to know with confidence that it is no big deal. so far, so good As long as runoff doesn't run through the bottom of other pots, or they don't sit in their own piss for long hours, the only loss is any benefit to a bit more surface area in which o2 can diffuse into rootzone at bottom vs just drainage holes. Larger the pot, the more important that'd be. Typical 10-12" tall pot may not be deep enough to make any real difference? Could probably find a quality answer to that with a web search. "elevated risers" are an option -- https://wholesalehydroponics.ca/images/product_variant/725675-01.jpg Garden trays have water channels, so a little slope/gravity can easily avoid runoff soaking back into any pot if it can drain away. diy - corrugated roof panels and a slope works too. The extra chunky stuff isn't something you can use mixed into substrate. So, it's one more bag of shit to store. i have really leaned into maximizing convenience lately. Avoiding the dust of mixing it together alone is enough for me. Mask or not, coughing up shit for days afterward. I do 10 pots at a time, so it adds a chunk of time to that process, but less so in any context with fewer pots. some people go the other way and get more extravagent. I attempt to do things that are only value-added and avoid the ocd stuff. Now, this particular issue isn't "ocd." it's important but can be done in more than one way and done well.

I do large chunk perlite personally https://ethosgenetics.com/resources/substrates-101

I like to put a layer of grow Stone in bottom and just reuse it. It's kinda pricey. But yeah putting something there helps with drainage. Same with having at least 30 percent course perlite in your soil mix. Help reduce overwatering chances as well. Roots like air as much as water, and nutrition.