From this page of the game guide, you will learn how to unlock the secret ending of Returnal, which provides a much better explanation for the game's plot.
In the game Returnal, in addition to the main ending, you can also unlock the second, secret ending. However, this requires quite a bit of work, because you need to thoroughly explore all six biomes of the planet Atropos.
First of all, make sure that you have played the 5 sequences connected to Selene's House:
- Overgrown Ruins: in the first biome, you can play 3 sequences;
- Echoing Ruins: in the fourth biome, you can play 2 more sequences, but the 3rd one will be blocked until the condition described below is met.
After killing Ophion, you will be able to watch the main ending of the game Returnal. When Selene once more wakes up in front of Helios, you should receive a trophy for completing Act 2 of the game and thus begin Act 3.
During the 3rd Act of the game Returnal, you will be able to freely explore all biomes. With the help of the device found on the Helios ship, you will be able to return to the 1st biome, the Overgrown Ruins, or go back to biome 4, the Echoing Ruins.
The trophy is prone to glitching out, so if you didn't get this achievement, you'll probably have to defeat the last boss one more time.
In each of the six biomes, in a well-hidden place, special items known as Sunface Fragments will appear. It should be noted here that it's not 100% guaranteed to come across the desired fragment in a given cycle, so investigate your surroundings thoroughly while exploring and if you fail to find this object, restart the cycle.
While hunting for the fragments, all the gadgets obtained during the course of the game Returnal will be extremely useful, i.e. the Icarian Grapnel, the Promethean Insulators, the Delphic Visor and the Hadal Ballasts. More information about these gadgets can be found on separate pages of this guide:
After collecting all 6 Sunface Fragments (you can check how many you have in the equipment screen), return to Selene's house in the Echoing Ruins biome and watch the final sequence. When Selene regains consciousness, she will be holding the car key that you might have spotted in the first ending of the game Returnal.
Now you have to once again go through the last biome of the planet Atropos and defeat the final boss of the game Returnal. Once the fight against Ophion is over, jump into the abyss, follow the light in the direction of the car and open the driver's side door - this will trigger the secret ending of the game Returnal.