Well back when Alien & Aliens were made, they were basically using all realistic effects: real objects, men in suits as the aliens, etc. In alien, alot of the ways they actually created the sets & props were extremelly primitive, in alot of cases even utilising household items to help create some of the sets or parts of the alien, amongst other things. For example, the panel on the side of the motion tracker was actually a painted ice-cube tray, the large vents on the walls in the tight corridors of the nostromo were actually the outer casing of a TV bolted to the wall, & the alien's tendon-like mouth parts were actually made from condoms that had been cut up & stretched to give it a certain look.
When Alien 3 rolled around in 1992 however, the movie industry was starting to use many more "effects" to create the illusion of what was supposed to be happening in the movie. So for the most part in alien 3, a rod-puppet was used for the creature's more complex movements such as running & the like, & in certain scenes, such as when Ripley's EEV is being hauled out of the crash site at the beginning, a matte painting was used for the background, although it stuck out like a sore thumb & looked quite out of place.
By the time of
resurrection in '97, CG had started to be utilised in movies much more, & as a result, alot of shots of the aliens were CGI. Some of these shots worked well & looked reasonable, although some looked absolutely horrid. Tom Woodruff Jr. was still playing as the alien for close-up shots in which suits were used, but compared to the first 2 movies where pretty much everything was done for real (using suits, etc.) rod-puppetry & shoddy CG just didn't quite look the part, even today CG hasn't been perfected, although it is much better than it was back then.