Nest no longer empty

Over a month ago I blipped an empty nest that is in a camellia just outside a bedroom window. I wondered at the time whether it was discarded or would it be reused.

Then a few weeks later, I spotted an egg in the nest. A couple of weeks later still, another egg, and now after another break, there are 3 eggs in there (although only 2 are visible here). I have never seen a bird in there at all - although I check at various times - including the night, and there is definitely no one sitting on them.

What I don't know and would like to know is - do birds lay their eggs over a period of time and then when they feel the time is right, sit on them? Or is this just a random 'got an egg coming where is the nearest nest' event'. I had sort of imagined a bird would lay a clutch of eggs and then sit on them consistently until hatched.

Will these still be hatched and will the first egg still be ok? And if not, why lay these eggs here over this period of time?

I am hoping some of the very knowledgeable folk here will be able to answer at least some of these questions.