The California Aqueduct delivers water from the western Sierras through the Central Valley and to Los Angeles. This is likely what NorCal refers to when they say SoCal is 'stealing our water'.

https://en.wikipedia.org/wiki/California_Aqueduct

Would be interesting to see the relative amounts of use by LA and by agriculture in the Central Valley though.

SoCal does, yes; about half the water going through the SWP from NorCal, or ~75% if you include Bakersfield/Kern as part of SoCal (though most would consider it Central Valley).

But SoCal isn't only LA. LA itself gets a bit less than half of their water from MWP, which manages the water from the SWP and the Colorado. About the same amount it gets from the the eastern Sierras. These are supposed to drop to ~10% of LA's water supply as recapture/recycling projects complete.

Or computed the other way around, LA only has rights to ~20% of the water managed by MWD. Of course water supply, distribution, and rights are all blended and traded around all the time, but generally speaking it's not "LA" using up that water from NorCal, the consumption is significantly more from the cities and farms that came after.

This infographic basically explains it:

https://www.ppic.org/publication/water-use-in-california/

tl;dr: Urban water use is tiny. In NorCal, the vast majority of the water flows unimpeded to the sea. In the Central Valley, most water is used for agriculture. Agricultural water use in any one of the 3 major basins in the Central Valley is more than all urban areas in California combined. Unsurprisingly, urban use is the primary one in the SF and LA areas, but the absolute totals are very small compared to total CA water supplies.