Hi Steve and welcome
Concrete floors are made for hygiene and to keep rodents and other pests from getting in via the floor. Basically it's so you can hose off bird poop and associated diseases & parasites. Drawback to concrete is it gets cold in winter so quail don't do well, and it doesn't look natural. But it is designed for hygiene.
You can leave it as concrete or put a thin layer (1/2 to 1 inch) of clean sand over it. Every month remove the dirty sand found under perches and put it on the rose garden. Then replace with clean sand. Alternatively, leave concrete and have trays of sand for the birds to have sand baths in.
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You mention one section is for finches. If you plan on keeping parrots in the aviaries next to the finch aviary, put a second layer of finch wire on the outside of the finch section. Have the second layer of wire about an inch away from the finch aviary. This will prevent parrots in adjoining aviaries from getting to the finches. Alternatively just rewire them all and have 11 finch aviaries, then go nuts
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A 4x4x2 meter aviary is fine for 7 pairs of finches. As mentioned by the others, just move the young birds out when the aviary gets a bit crowded.
Having plants in the aviary will make it look more natural, provide shelter, resting and nesting sites, and break up territories. They do however, make it more difficult to catch birds. If you have plants in 30-40cm pots, then you can move the potted plants out of the aviary, catch the birds, then move the pots back in.
Make sure all the nests are the same height and have a bit of space between them, and you should be fine.
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Having an open flight section is fine in dry weather, but it should be covered during the cooler seasons. Some people use clear plastic in winter, others use clear or tinted laserlite/ polycarbonate sheeting, and others use fibreglass sheeting. Tin sheets use to be the norm and still is for the main aviary structure. Unfortunately tin gets very cold in winter and hot in summer, and makes a lot more noise when its raining. This can disturb the birds at night if its raining or hailing. However, tin is stronger.
Personally I like tin on the outside of the main shelter with a 1 inch polystyrene foam sheet glued to that, and then fibreglass panels stuck on the inside. It insulates from hot, cold and noise. The fibreglass sheeting is also easy to clean but is not recommended for parrot aviaries because they chew it.
For flights: in summer I had 75% shade cloth suspended about 6 inches above the aviary and sealed up all around the sides to stop cats and vermin getting under the shade cloth. In winter I remove the shade cloth and replace with lightly tinted polycarbonate sheets that are in panels that can be bolted onto the roof frame.
I like clear panels on the roof but there are too many cats around here that sit on the aviary, and the tinted panels make it harder for the cats to see the birds.