It seems the best light for starting baby plants is sunlight

Not necessarily. Plants only need full spectrum light to bloom and set fruit, not for leaf growth. And young seedlings can easily be killed by direct sunlight, especially by UV. You need only look at all the posts about sun scorched plants here.

Then there is the air/soil temp factors. Full sun exposure presumes heated air. Excessive heat when growing young seedlings results in leggy plants. Cool soil temps can result in stunted plants and poor root development.

So "best"? Not in terms of development or survival rate. Plus if one considers the delay in starting the plants while waiting for the weather to allow it, for the soil to warm sufficiently, the planting/harvesting season would be greatly delayed.

If what you describe had ever proven to be the best way to do it then everyone including commercial growers would be direct seeding their tomato plants instead of using transplants. Rather they have learned from decades of experience that it isn't the "best" way for many reasons.

Dave

So you think it's better to start them inside under grow lights even when it's warm and sunny outside?

As soon as all of my seedlings have emerged, I take them off of the heat mat.

They go straight outside if the weather allows during the day, and into the garage at night. On particularly cool nasty spring days, they stay in the garage under grow lights during the day.

I like to expose them to full sun and weather from the beginning, before any true leaves form. This way the plant grows pre-hardened off.

Of course, I am growing about dozen plants I can easily move back and forth. If you had a large number, this may not be practical.

So you think it's better to start them inside under grow lights even when it's warm and sunny outside?

The ideal way is to start them in a greenhouse where they have all the advantages of limited spectrum, UV blocked sunlight (and supplemental light when needed) but in a temperature and air circulation controlled environment.

But that isn't practical for most small home gardeners. So yes, the next best method is indoors under lights and fans until they are established and old enough to be gradually exposed to outdoor elements.

Not only does it give you a head start on the season but it results in transplants that, if it is done properly, are almost as good as greenhouse grown plants. The problems with transplants that can result from growing indoors under lights are due to the grower not understanding how to do it properly not because they are grown indoors.

Dave