How to create a watercolour splash effect

Luckily I do not have  half a brain, so let me try to help you.

Yes these brushes are images, but I suppose the Watercolor brushes in Illustrator started out that way.

Have a look at the brushes that are part of Illustrator:

Brushes Panel > Open Brush Library > Artistic > Artistic Watercolor

Drag them  from the Brushes panel to your artboard to see what they contain.

Most of them are groups with reduced opacity.

Double click them in the Brushes Panel to see their options.

Learn how to create them from scanned brush strokes with Image Trace and shades of gray.

Be careful when using them, the resulting file can become very complex and will sometimes only print if you create a rasterised version,

I don't want to buy the vectors as I would like to know how to create them.

...only that they aren't vectors at all? it clearly says that they are high-res scans of actual color blots. Anyway, such stuff is either nearly impossible to create as vectors or the many fine details required will cost lots of time and gobble up your computer's resources. You can't find any tutorials because anyone with half a brain would not do it with vectors unless being paid a lot of money...

Mylenium