The Somnaran Sail's main purpose is to spread life, which includes replicating itself on other worlds. The ship drops Somnapods onto other planets- these pods then analyze & repurpose the planet's resources and use them to replicate itself into a new vessel.
Here we see a scene where Humanity encounters a Somnapod for the first time.
The Somnaran Sail was first discovered by Earth scientists when it entered the Orion Nebula. It was instantly classified as a threat, but soon showed that it's main purpose was to create and spread life throughout the galaxy.
The solar sail itself absorbs light through chloroplast cells embedded in the sails skin. This energy then spreads itself throughout the vessel using the complex vein-like structure until it has been distributed evenly.
The US Navy detects underwater radio patterns coming from a crevasse deep in the ocean- they send a scouting team, and uncover a lost artifact from a forgotten age. After the Navy learns of the ships existence, they send a team to uncover and retrieve it.