The SEI layer (film) is porous so it can pass small Lithium ions, but at the same time the holes are small enough to prevent further reaction of larger solvent molecules with the active material.
SEI film consumes Lithium, thus reducing the capacity of the cell.
The goal of Cell formation is crating a thin but uniform SEI layer on the anode particles.
SEI layer can break (crack), and new SEI is formed in the cracks.
SEI layer grows because of Coulombic efficiency less than 1.
SEI layer is exponential: the power is proportional to the negated potential difference between the electrode and the electrolyte and is proportional to the temperature.
See Tafel equation for more.
SEI layer growth faster when we at a high state-of-charge (because the potential difference between the electrode and the solution is smaller, probably), and also faster when we are charging at high rate.
Time, not cycle count is the dominant factor of SEI thickening.
SEI growth is one of the main mechanisms of the Loss of lithium inventory.