I never sit by ability. I couldn't the first couple of years because my classes meant I had to sit by behaviour anyway! If I'd sat by ability it would have been chaos. We had rows for 6 months! I colour-code my books to help me manage my marking, but the kids see the list at the beginning of the year when we sort the books, so they know it's just alphabetical.
I explain things to everyone, get them to tell me (on thumbs or whatever) how confident they feel, then pull any that need it to the carpet for extra modelling or practice before they try it independently. Sometimes I'll pull kids even if they said they were confident because I know they'll need help, and sometimes I'll send the ones who want help all the time to their table so they can challenge themselves without hand-holding.
Everyone does the same work. Very needy children will still have the same thing, for the most part, but might break it into stages and only do one. Eg. column addition broken into parts.
We do go into sets for maths (mixing our three classes), but I still have to do this in my group as there's a lot of difference even when you set. There's a lot of variation in my group depending on the topic, so I have some who are amazing at arithmetic but can't do reasoning etc. I then have a focus-table for them where I or a TA sit and support, but this changes all the time depending on the topic. There is challenge built into all the work, and sometimes this is helping or explaining to others - it's actually a useful skill and shows depth of understanding!
When I got my first class they had been on ability tables the previous year, and they were really nasty about it - some of them caused a proper fuss about not wanting to sit with particular children because they didn't want to be on 'that' table. I squashed that very fast, but it had had a horrible effect on the class. They didn't want to be friends with people from 'the red table', because that was the 'stupid' table.