If we wanted to actually see evidence of artificial "things" on exoplanets around Alpha Centauri, at 4.2 light years away, with optical telescopes, we would have to make assumptions about how big those things are. Let's say agricultural constructs are the largest, at 1-km in size, although that might still be too big. We would need an imaging interferometer with a baseline equal to the Earth-Moon L4/L5 Lagrange Point separation, and a collective light bucket of 57 square km.
Unfortunately, sensors that can record both amplitude and polarization of light in the visible spectrum don't yet exist, so an imaging interferometer comprised of multiple space telescopes isn't yet feasible. So, we must wait for this new sensor technology to be invented.
Unfortunately also, there is only one star system at a distance of 4.2 light years. If nothing is found there, then achieving the same result for more distant star systems requires a much larger baseline separation and light bucket.
If we want to survey the closest one million G-type star systems for exoplanets with artificial structures 1-km in size, then we need to see out to a distance of about 1250 light years. The Sun-Earth L4/L5 Lagrange Points provide ample baseline separation for 1-km resolution at 1250 light years (again assuming someone invents sensors for optical imaging interferometers). However, the required light bucket is immense, coming in at a staggering 5 million square km, equivalent to a single aperture about 2510 km in diameter. For comparison, Earth's moon is 3474 km in diameter.
I'm not saying it's impossible, but at the current Kardashev Level that humans occupy, it's aspirational at best. That is, it's not a fantasy that violates the laws of physics, but it is definitely science fiction.
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