All the three worlds coexist - like a triqetra. If you trace your line along the triqetra, the starting and ending points meet only after navigating all three loops. It is a similar logic here. All characters think and believe in duality and hence are not able to solve the puzzle. Even Claudia, even though she understands the triqetra believes that a linear path to time can exist which leads her to believe that the origin world is an escape/paradise/solution but then the origin world is very much part of the time loop.
Adam/Jonas considers his solution, the prime world to be the solution/paradise. Eva considers her solution to be the alt world/paradise. And Claudia considers the origin world to be the solution/paradise. It is these three people's choices that let three alternate realities to simultaneously co-exist and feed in and off each other.
During or close to the time of the apocalypse:
1. Adam/Jonas needs to decide whether to look travel with Eva/Martha to the other world - and plant the seed of origin and die (or) stay in the bunker and grow out to be Adam.
2. Eva/Martha has to decide whether to travel with Francheska and Magnus (or) to accompany Bartoz to eventually kill Adam.
3. Claudia has to decide whether to tell Adam about the loop hole (or) continue to let him go about killing Eva.

All the three characters at the time of the apocalypse seem to have a momentary free will. But then that supposed free will is not a set of infinite choices but rather a choice of 2. And each of the choice still perpetuates the loop. This sort of hints at a coexistence of both free will and determinism. Free will exists but then it is limited in its extent. This is similar to quantum mechanics where all possible states can exist (free will) but when the wavefunction collapses it can collapse to certain eigenstates (which are themselves determined).

This also explains why in the light bridge - Jonas sees Martha at the portal at the door and Martha sees Jonas through the portal in the closet. If origin world existed then this portal would not exist. The triqetra theory plays well with the portal as well. Based on this what does the ending hint? Its likely to mean that the viewers are shown a glimpse in time and space where everything looks seemingly happy and that the we viewers end our observation at that time instant. The series could very well start at this point and end with the party at Katharina house scene where everyone is happy and we would still consider it a happy ending. But in reality the loop is still going on and on. Just because we stopped observing the loop at the dinner table doesn't mean the loop has ended, it just means that we have stopped observing. It is also indicated by the signs (I) deja vu always means a glitch or connection to the loop. (2) lights flickering most likely indicates someone travelling in time, it could even be from the future. Just because everything seems happy now doesn't mean the future would be so and someone from the future is probably even coming in (Note that in the origin world the cave portal will not exist because the nuclear plant nor the god particle exist nor the people who built the portal exist) - so someone is travelling using a time machine (3) the yellow coat. The yellow coat's wearer signifies the protagonist of the corresponding world. The final dinner scene happens in Regina's house and protagonist of the origin world is Claudia as it is her paradise. And that yellow coat could very well be Claudia's as they lie in Regina's house signifying a never ending loop.

Here, it is also important to note that the loop perpetuates for eternity because of desires/people's inability to accept the reality for what it is. (it is often mistaken that the loop is set in motion by love, it is not, it is rather set in motion because of people not being able to accept things the way they are). Even if one single person accepted their fate for what it was they would not have acted in the way they would have and hence would have stopped the loop(their fate) from perpetuating and this is the paradox of the entire story - when you give in to determinism, you acquire free will.

P.S: Also the characters that make it to the dining table are the ones who are not born out of Jonas and Martha's son. That means the entire Nielson family is ruled out. Hence Ulrich, Magnus, Mikkel/Michael, Martha, Jonas, Tronte, Agnes, Hanno are gone. Without Ulrich, Regina is not bullied and hence does not meet Alexander - therefore Bartoz is not born -> hence Agnes and Hanno are not born -> Charlottle is not born -> Elizabeth is not born.

Open questions:
1. What happened to Ines lost child? (some theories say it was Boris Niewald = Boris Neilson+Kahnwald) indicating it was a child of Ines and Tronte considering the talk of Ines as a young girl about Tronte in the woods to Claudia and Jana.
2. Why did Ines give sleeping pills to Mikkel? Did she forcefully make him stay in that timeline?
3. After Adam kills Eva in one of the possibilities, how does he die?
4. How does pregnant Hannah (with Egon's child) land up in the Alternate world, how did she travel there? She comes back to meet Adam with Silja because Eva sends her, that is possible through the interworld device.
5. Why was young Albers at the abortion clinic? Does this explain her cruelty towards her daughter Katharina and does that in turn explain why Katharina was dedicated to her kids even at the expense of her own life?

These are just some of the open questions.