Everyone is different, so if you’re asking this question regarding what YOU can do, I can’t answer that. What I CAN say is what I’ve seen and experienced. I’ve always been lean, and probably not been above about 14% bodyfat at any point in my entire life. I’ve spent most of the last 6-7 years in the 8ish to 11% range. I don’t vary much, but I also don’t actually measure my bodyfat. This is a relatively good estimate though. I can consistently see a lot of veins, my abs are clearly defined, etc.
I was able to do this when I was natural, and it’s a little bit easier to do now that I’m not.
6% all year round is unlikely, but under the right circumstances it can be done. The trouble with this is that gaining muscle while maintaining that level of leanness, literally year round, is almost impossible, and certainly isn’t going to happen if you don’t have incredible genetics for it. I’ve known exactly 1 person in my life who I thought could maybe stay in this range and even gain muscle at it. Literally 1 individual.
Here’s the problem with the question you’re asking though. As far as I can tell, you don’t have a real world application for the answers you may receive by asking this question. Are you just trying to figure out who is and isn’t on steroids? That’s not a great path to go down. Are you just asking for yourself to figure out your own expectations? If THAT’S the case, then my advice would be to learn everything you can about diet, and apply it to your own life, and FIND OUT how it works for you. There is no reasonable way for me to tell you what you can do, but you can certainly learn it on your own time with your own body.