ive fallen down many a rabbit hole on those large, religious families and i've observed:
housing - they live in cheap areas of the country. Anna Duggars brother bought his his for $40,000 according to public records and his MIL said they were debt free. The men start working quite young and are expected to save money before they get married.
food - the families i've seen do eat cheap, processed foods. Jill Rodrigues has posted picture of her meals and based on volume, portions would be tiny for 15 people.
second hand - they buy used and the stuff they buy is used multiple times. Joy anna Duggar was going on a date with her now husband and was searching a mountain of shoes in her house to find a matching pair (she didn't care what ones).
Technology - they don't let their children have phones, limit TV and internet, so they spend very little on technology. they homeschool, so the children don't know any different.
holidays - often spent visiting like minded families and bunking in with those families, or sleeping on their converted coaches.
the families don't have to pay for labour. The Duggars 'surprised' the Bates family by building them an extension and all the boys and girls picked in with the building and decorating (Jill Duggar did the tiling).