The most crucial thing for them is they have to beg for food. Since they have abandoned fire they are not allowed to cook for themselves. So, the only option left for them is to beg and secure food. That is the only method by which they are allowed to secure food.
And, for that purpose, some of them use a bowl (which they were presented with during renouncing) whereas some simply use their hands as bowl.
Let him then take nothing besides a loincloth. "Thereafter they live
the life of mendicants. Celibacy, noninjury, non-possession, and
truthfulness—guard these assiduously."
"Now, Paramahamsa renouncers sit and sleep on the ground. Celibates
use bowls made of clay, bottle-gourd, or wood, while ascetics enter
a village to beg for food, using their hands or stomachs as begging
Aruni Upanishad 12
The amount of food that is prescribed for them is usually just eight mouthfuls.
He may beg food from all four classes. He should take his food using
his hand as a begging bowl, Let him partake of food as if it were
medicine; and let him eat whatever he happens to get merely to sustain
his life, so as not to put on fat and to remain lean. Let him spend
just one night in a village and five nights in a town. He may spend
the four months of the rainy season in a village or in a town
Kathasruti Upanishad 32-33
There are also restrictions regarding the number of houses he can beg from. This style of begging is called Madhukara Vritti (the style of the bees) ( taking little little amount of food from certain number of households).
"A Bahudaka wears a topknot, a patched garment, and so forth, as well
as a triple mark on his forehead, being similar to a Kuticaka in all
things. He eats eight mouthfuls begged in the manner of a bee. "A
Hamsa has matted hair and wears a triple and a vertical mark on his
forehead. He eats food begged in the manner of a bee from houses that
he has not preselected, and he wears a rag as a loincloth.
Turning once again to the distinction among ascetics, a Kuticaka begs
food from a single house. A Bahudaka begs in the manner of a bee from
houses that he has not preselected [NpU 174, n. 59]. A Hamsa begs
eight mouthfuls from eight houses. A Paramahamsa begs from five
houses and uses his hand as a begging bowl. A Turiyatita eats fruits
and uses his mouth in the manner of a cow . An Avadhuta obtains food
from all classes in the manner of a python.
Narada Parivrajaka Upanishad
He obviously can not demand so he does not have any control over the kind of food he will receive.
Basically, he has to eat very moderately and also has to avoid consuming certain food items like Ghee specifically.
Let him beg food, avoiding those who are infamous or fallen from
their caste, heretics, and temple priests. In times of distress he may
beg from all classes.
Ghee is like dog's urine, and honey is equal to liquor. Oil is pig's urine, and broth is equal to garlic. Bean cake and the like are cow's
meat, and milk is equal to urine. With every effort, therefore, an
ascetic should avoid ghee and the like. Let him never eat food mixed
with ghee, broth, and the like.
Brihat Sannyasa Upanishad