Read the article. Her voice was heard on every episode, but recorded separately. So, for example, the cast of TNG never met her until she showed up for her first guest spot as Lwaxana. Sirtis relates their (or her) initial reservations:
I didn’t meet Majel until she was a guest star on our show in the episode ‘Haven,’ and we didn’t bond immediately because she was the boss’s wife, and we were all a bit nervous because as a cast; we were a little rowdy
Okay, that was rude and nitpicky. But I’ll bite:
Read the article. Her voice was heard on every episode, but recorded separately. So, for example, the cast of TNG never met her until she showed up for her first guest spot as Lwaxana. Sirtis relates their (or her) initial reservations:
Note the distinction I put in italics.