Aliens...why do so many just assume?

Unless you’re a member of the crew, traveling at 99.999% c, will afford you some really cool time-dilation effects from what I hear.

Laser or microwave pushed sails aren’t “magic technology”. And in-system, once you build the infrastructure you can use spinning tethers as kinetic storage, magnetic mass drivers, etc.

Wouldn’t there be a certain range of effectiveness, there? Also, how are you supposed to slow down or even stop once you enter another star system?

Yes, there would be a range. As for stopping, someone came up with the trick of making the sail’s center detachable; at the deceleration point, it breaks free and continues to be accelerate forward, in the process bouncing the propelling beam backwards. The payload and a remnant ring-shaped sail catch the backwards beam and use that to decelerate.

And that’s only for the first vessel, probably a probe. By the time we or some alien species started serious interstellar exploration, it’s to be expected we’d have the technology to send a single machine forward that once it stops, could replicate and then build the facilities necessary for another laser/maser.

Another potential means for filling the galaxy is the “oasis” method. Where independent, mobile fusion powered space habitats move relatively slowly from asteroid to comet, outward from the Sun, essentially colonizing space itself rather than planets. Moving from one comet or rogue planet or whatever in interstellar space like desert nomads travelling between oases.