Where are they to be kept? That's the beauty of it Senator...They don't need to be "kept". And hotels are a lot cheaper than detention centers.
They need to be processed and sent on their way. Photos and fingerprints taken. Identities verified whenever possible and background checks done. Issue them photo IDs that will serve also as balance transfer cards so that employers can pay them through the immigration system, the cards will also track them wherever they go and automatically collect taxes for fees and services.
A matter of minutes in most cases days in others. Hire people to process them rather than people to terrorize them.
Think of these people as volunteers who have come here to make America great again!