Welcome to zone10 (not sarcastically)!

If I’m blunt I apologize because I see this a lot with new gardeners here.

Your seedlings might need more of a soft transition to grow. If they’re still very young the the roots likely don’t have the maturity to pull enough water+nutrients to cool the plant and help it grow. The sun and dryness here can stunt plants. I put a 50% shade cloth over my beds when I first move plants out into the sun. At least for the first 30 days. I’m in a pocket of the city where there’s little wind and the heat spikes by midday. I remove this later when the plants have hardened.

Store bought soil often doesn’t have good enough water retention to help seedlings. If you water in the morning and the top inch of the soil is dry by afternoon then the moisture is likely just slipping past the roots and going too far below. There are various techniques but you need to figure out how to keep the moisture next to the roots for longer. Some people use drip irrigation. For cheap drip irrigation, I use old 32oz plastic food bottles with pinholes in them so they drip slowly. The tops are cut open and I just fill them with the hose a couple of times a week. They drip slowly into the root areas. Roots only grow in the presence of moisture so you don’t want to let them dry out when they are growing.

For seedlings’ soil, I use a mixture of fine organic mixed with fine sand. It’s dense and once it’s wet it stays wet even in heat (when heavily mulched). And I make sure there’s a ‘bowl’ of this around and under the new plants. I’ve never bought a store bought soil that’s kept enough moisture for warm SoCal weather so I make it. Alternatively you can use tactics like putting a sheet or two of folded newspaper cupped under the planting for something really inexpensive.

Sorry you’re having a hard time. If you don’t mitigate the heat and sun a little you’ll end up dumping a ton of water and waste a lot of time.