When Xenogears was first made, it was supposed to be "Chapter 5" in a six chapter story. What the other five chapters were supposed to be was outlined in a book only released in Japan.
When the people who developed the game left Square and formed Monolith Soft, their initial plan was to mostly continue the originally intended story. Xenosaga was going to be Chapter 1, the next game was going to be Chapter 2, and so on.
Plans changed almost immediately - they wound up skipping Chapter 1 entirely (it's referenced in the opening scene flashback with the astronauts), and Xenosaga basically became Chapter 2. And then they kept releasing direct sequels to it, rather than moving on to Chapter 3. And they kept changing minor details along the way, weakening the overall connection to Xenogears.
By the end all three Xenosaga games basically wound up being a sort of different parallel universe, rather than a direct prequel to Xenogears. So in that sense, it's technically not really a spin-off any more.
even though it do show the 2 main char from xenogears on xenosaga? they show up as children but you can totally see who they was
xenosaga and xenoblade are derivated from xenogears, so it can be considerated a spin off or spiritual sucessor ---