Sometimes I sing from memory, sometimes from sheets (even if I wrote it - complexity is a rod for my own back)
The best approach that I know is, if you DO have to have it in front of you, sort your material out in advance and collect together what you need, in the order you need it.
That way you don't have to rustle paper during anybody else's performance and don't waste the audience's time hunting through a lever-arch binder.
As for those who memorise everything, either (a) congratulations on your brain capacity or (b) lucky you for having the time - some of us have day jobs and have to spend most of our "spare" hours pushing back against the entropy of life-in-general like home and car maintenance, gardening, cooking, shopping etc. so cut us some slack.
Maybe when I've retired, I'll memorise everything, too. Until then, there's paper to fall back on. Or a 12" Android tablet with slow-scrolling PDFs....