It's just another way of saying markets. In Spain at least, you often hear old ladies saying they are going to "la plaza" when they mean the market.

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Note added at 22 mins (2019-10-23 08:55:24 GMT)
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Wiki: Se denomina mercado de abastos o también plaza de abastos a unas instalaciones cerradas y normalmente cubiertas, situadas en las ciudades donde diversos comerciantes suministran a los compradores todo tipo de perecederos como carnes, pescados, frutas y hortalizas. También pueden existir otros comercios que venden pan, lácteos,flores o alimentos en general así como diversos artesanos. Las plazas de abastos modernas disponen de cámaras frigoríficas para conservar los alimentos perecederos.

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Note added at 27 mins (2019-10-23 09:00:33 GMT)
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But if you need two terms (although I don't see why), you could use "food halls" for the fancier, more upmarket type of markets, like La Boquería in Barcelona or San Miguel in Madrid, where they have little gourmet stands, etc.

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Note added at 28 mins (2019-10-23 09:01:40 GMT)
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in reply to your note re indoor and outdoor, I think the outdoor type would be covered under mercadillos ambulantes.

«instalaciones cerradas y normalmente cubiertas, situadas en las ciudades donde diversos comerciantes suministran a los compradores todo tipo de perecederos como carnes, pescados, frutas y hortalizas. También pueden existir otros comercios que venden pan, lácteos,flores o alimentos en general así como diversos artesanos».

I think it comes down to how your text is categorising them. It seems in this particular line that they're being categorised both by the permanence of their location, and their physical structure. So you may want to differentiate the "fixed" aspect of "mercados" from the physical aspect of "plazas de abastos" (i.e., covered).
I think you could therefore use "fixed markets and market halls" as a neat way to get around the indoor/outdoor conundrum because "mercados" aren't necessarily outdoors, yet they are in a fixed location, and particularly as "mercadillos ambulantes" can be both of the outdoor and indoor kind.

"A Market hall is a covered space or a building where food and other articles are sold from stalls by independent vendors."
https://en.wikipedia.org/wiki/Market_hall

You could then perhaps use "itinerant markets" for "mercadillos ambulantes".

"In different parts of the world, a market place may be described as a souk (from the Arabic), bazaar (from the Persian), a fixed mercado (Spanish), or itinerant tianguis (Mexico), or palengke (Philippines). Some markets operate daily and are said to be permanent markets while others are held once a week or on less frequent specified days such as festival days and are said to be periodic markets."
https://en.wikipedia.org/wiki/Marketplace