You have to read up on what was happening with each baby and how she ended up in the room time and time again with a collapsing baby the minute other staff members or parents left.
That's a common misunderstanding, @Firefly1987 . On some occasions she was unable to prove she hadn't been in the room with collapsing babies the minute other staff members and parents left, because the parents and staff weren't there to witness whether she was there or not. So the prosecution assumed that's when she attacked. It helped that their experts refused to be pinned down on timing - an air embolism or air in the stomach could take effect whenever suited. And how do you prove you weren't somewhere, aided by occasional medical notes, a decade later?
In other cases of course there is no dispute. Other people were in the room with her and testified accordingly. That's the case for babies A, E and P, for what were supposed to have been the moments she murdered them. Witness accounts (not just Lucy Letby's) are contradictory to put it kindly for babies C and D. Babies I and O didn't collapse immediately after anyone left them with her either. So it doesn't work for the deaths.
Of the collapses she was convicted of - I'd have to refresh my memory for some but it's not even claimed for babies F, L or M, and the prosecution had no evidence she was in charge of baby N when he collapsed. They worked backwards, assumed he was harmed, and therefore assumed she was the unknown nurse babysitting him.
People assume this was all pieced together carefully because how else would we know she killed them? The bad news is, it was exaggerated to the point of invention in some cases, and the logistics were just ignored in others. The closer you look, case by case, the worse the conviction looks.