Vaginal lubrication isn't caused by a single thing. There's lots of processes going on which make the vagina a moist environment. Even when you're not turned on, the vagina is usually fairly damp.

Outside of arousal, most of the wetness in the vagina comes from the cervix. Cervixes contain glands that secrete mucous. The amount and texture of the mucous varies at stages of your cycle. You can see this in action here: https://masto.ai/@vagina_museum/110571170513632506

As well as cervical mucous, the vaginal flora - the natural colony of bacteria in your vagina which keep it clean - are secreting lactic acid and hydrogen peroxide. There's also a type of fluid called transudate, which comes from the blood vessels and is similar to blood plasma.

These natural secretions leave the vagina as discharge, and have a natural pH of around 4, though it varies with your cycle.

When you're turned on, more lubricating mechanisms kick into action. Increased blood flow to the vagina results in increased production of transudate. This creates a slippery, thin fluid which lubricates the inside of the vagina.

Glands on the vulva also play a critical role in producing wetness, lubricating the entrance to make things even slipperier.