Basic seed sowing instructions.
Simon Speed via Wikimedia / CC0 1.0
If you want to be able to leave your seedlings in their containers for longer to give you a more flexible planting out/potting on date, then when you are filling the containers before sowing:
The seedlings then germinate in the compost without being affected by the fertiliser below (which could prevent germination), grow their roots down until they reach the fertiliser and can remain growing in that container for a whole season, if necessary, as the fertiliser provides the nutrients they require.
Seed sowing by hand is labour intensive and so often not viable for a commercial enterprise.
Search on YouTube for ‘seed sowing machines’ and watch how commercial nurseries speed up the process.