I don't know exactly what Sagan was trying to imply aside from there being a long chain of events that lead to the whole that is now. According to some in the cosmology and physics community, it is possible to create your own universe, but I doubt you'd be making it "from scratch" because you'd just be creating it from a preexisting, eternal sea of infinity which prevents creation ex nihilo, in its most literal from, from occurring. I don't know if Sagan was aware of any of this or not because it seems the earliest speculation of unironically creating a universe was 10-20 years ago. If you really want to make something from scratch, taking it to its logical limit, you have to make 1 from 0, but 0 is perhaps the only concept that has no manifestation in existence because there is no such thing as "nothing."
When scientists say the universe came from nothing, they want you to know that "nothing" is an impossibility. [Pun intended.] In other words, 'nothing' is a meaningless word that misleads people, specifically when used to describe what existed before the visible universe started to expand, because there is always going to be a energetic foam of possibility that spawns an infinite number of universes via endlessly selecting random values that correspond to physical constants. An infinite number of these spontaneous creation events will yield invalid universes which couldn't ever produce anything interesting (by human standards), whereas an infinite number of them will actually produce stuff like stars, cows, humans, and apple pies.
The best way to grasp it would be to replace the word "nothing" with the phrase "nothing on average." There are places where "nothing on average" exists (aka everywhere), but there are no places/times where/when "nothing forever" could exist. You can say this is all one massive word game, and you'd be right and wrong simultaneously; the only thing that exists is words because everything, including us, is made of an evolving information structure that is equivalent to a human or mathematical language in the process of growing and evolving. The only difference is one is eternal (never halts), and the other is eternal on average (will continue forever, but in distinct chunks of cyclical periods).
Imagine yourself navigating the perimeter of a Koch Snowflake: you can progress along the perimeter of a snowflake for an infinite amount of time at an infinite speed, but even if you actually did this successfully, you would have made zero progress in total because the perimeter of the snowflake is infinite (both as a whole and in distinct, finite chunks). In this framework, 0 and 1 are equivalent to each other because no matter how you count, move or progress, progress is an impossibility because you are stuck in this hell, so kill yourself now faggot. Oh wait, you already did, and that's why you're here now. If you fully understood what is going on here, you'd be totally confused, but that confusion is understandable because it is understanding that is confusing.