I always sow my carrots on the top of the soil and tamp them in, water until they are up. I thought everyone is doing that. But I just read in a well known gardening book (the one from diggers club what new is old or similar) that carrot seeds can be sown up to a cm deep?? Is that true? Then it would be easier with watering, but usually I have good success with carrots.
I do rake lettuce very slightly in.
Are there veggies which have to be sown on the top? What's with basil?
Carrots benefit from even moisture when germinating (and don't need light). I've learned from others to sow shallowly, water in and then lay a board on the surface of the bed to keep things damp and moist. After a few days, I start to check for seedlings, and remove the board as soon as they are seen. These sorts of seeds tend not to persist for years in the soil; for them it's go and grow.
Lettuce seeds (and many other kinds of small seeds) require for light for germination, so sowing them shallowly (and not covering) is important. Even moisture can be harder to maintain but it still helpful! Note that many weed seeds fall into this category; they lay dormant in the soil and only germinate when brought to the surface (hence one advantage of no-till).
Large seeds have greater stores of food and generally benefit from being sown more deeply. Beans, peas, corn etc here. For these sorts of seeds, the greatest risk is being eaten before they germinate!
There is a normal rule for sowing seeds which is; small seeds are sown shallow, large seeds are sown deep.
In the real world any small seed can be top sown and then pressed in or it can be "sprinkled" with soil or it can be sown in a shallow trench.
This goes for any small seeds. Basil is a small seed, so the same rule goes.
The best method is to plant at a depth that is two times the diameter of the seed.
That means something like a radish will go very shallow while something like a pumpkin will go much deeper.
Using the 2x diameter for seed planting depth has never failed me.