theres way too many possible scenarios

scenario A) theres some basic blight (deteriorating brick, missing gutters, bad roof) etc on an otherwise occupied and mostly "ok" property. in this case, they get cited by the building department, the owner makes the needed repairs, and the issue goes away. this is assuming they have the funds needed to make the repairs (in many poor neighborhoods they dont, which is why theyre in a state of dis-repair in the first place)

scenario B) the property is abandoned or in severe disrepair. in this case, it is a long long process involving building court. likely its a result of a foreclosure or extreme neglect. in some cases, the owner walks away from the property entirely, and in these cases the condition of the building declines rapidly as its exposed to the elements, gangs, etc. first the owner will get cited over and over, and typically there will be an active case against them. over a period of years, assuming nothing changes, the city will acquire the property through a court case. typically, the building is far too deteriorated to save without significant cost. so what will happen is the city will demolish the property, and then try to sell the vacant lot or re-develop it.

the sad reality is the neighborhoods where this is occurring often dont have strong demand in the first place because they are plagued by poverty and extreme violence (and also hyper segregation). so it creates a viscous cycle where the city is demolishing thousands of buildings, which makes it even more difficult for the neighborhood to rebound in the long run. in most cases, the cost to restore the building would cost more than you could otherwise be able sell it for in that neighborhood and theyre structurally compromised. in the cases where you CAN sell a rehabbed building for more than you acquired, well those neighborhoods are already undergoing gentrification and general improvement of the existing housing stock.

what Chicago is trying to do is tax developers building new construction to allocate a certain amount of money to an affordable housing fund. they then are taking those proceeds and trying to put up city-driven developments on some of those vacant lots. theyre also trying to offload a lot of those vacant lots into the hands of organizations who are willing to put up affordable housing or to do redevelopment. theres a lot of info here about the different approaches:

https://www.chicago.gov/city/en/sites/invest_sw/home.html