The Enterprise computer was (usually) portrayed as fairly close to what we have now with today's "AI": it could synthesize, analyze, and summarize the entirety of Federation knowledge and perform actions on behalf of the user. This is what we are using LLMs for now. In general, the shipboard computer didn't hallucinate except during most of the numerous holodeck episodes. It could rewrite portions of its own code when the plot demanded it.
Data had, in theory, a personality. But that personality was basically, "acting like a pedantic robot." We are told he is able to grow intellectually and acquire skills, but with perfect memory and fine motor control, he can already basically "do" any human endeavor with a few milliseconds of research. Although things involving human emotion (art, comedy, love) he is pretty bad at and has to settle for sampling, distilling, and imitating thousands to millions of examples of human creation. (Not unlike "AI" art of today.)
Side notes about some of the dodgy writing:
A few early epsiodes of Star Trek: The Next Generation treated the Enterprise D computer as a semi-omniscient character and it always bugged me. Because it seemed to "know" things that it shouldn't and draw conclusions that it really shouldn't have been able to. "Hey computer, we're all about to die, solve the plot for us so we make it to next week's episode!" Thankfully someone got the memo and that only happened a few times. Although I always enjoyed episodes that centered around the ship or crew itself somehow instead of just another run-in with aliens.
The writers were always adamant that Data had no emotions (when not fitted with the emotion chip) but we heard him say things _all the time_ that were rooted in emotion, they were just not particularly strong emotions. And he claimed to not grasp humor, but quite often made faces reflecting the mood of the room or indicating he understood jokes made by other crew members.
It's the relatively crummy season 4 episode Identity Crisis, in which the Enterprise arrives at a planet to check up on an away team containing a college friend of Geordi's, only to find the place deserted. All they have to go on is a bodycam video from one of the away team members.
The centerpiece of the episode is an extended sequence of Geordi working in close collaboration with the Enterprise computer to analyze the footage and figure out what happened, which takes him from a touchscreen-and-keyboard workstation (where he interacts by voice, touch and typing) to the holodeck, where the interaction continues seamlessly. Eventually he and the computer figure out there's a seemingly invisible object casting a shadow in the reconstructed 3D scene and back-project a humanoid form and they figure out everyone's still around, just diseased and ... invisible.
I immediately loved that entire sequence as a child, it was so engrossingly geeky. I kept thinking about how the mixed-mode interaction would work, how to package and take all that state between different workstations and rooms, have it all go from 2D to 3D, etc. Great stuff.