The mechanical pieces in the synths (strider, gunship, etc) are so flush with the body that I've been really bothered with how they are made. Are they born, are they built?
EDIT: Here are the sources I think I was looking at. Unfortunately they are fan-made videos, so not entirely credible, but still interesting and they seem factual. Game Theory:
Top 10 Facts:
To add information: the Vortigaunts were forced to flee from their homeworld because the Combine took over and enslaved them. It seems that the Xen creatures (headcrabs, etc) were already on Xen - it is their homeworld.
This here will also be useful to read - it's the Wiki about Synths. It says there is nothing that has been confirmed that they have been enslaved, but there are sources (such as Ep 2, the strider's brain and the Hunter's blood) that supports they are living
From what I have read, and I'll try to find sources, the Synth creatures are slaves to the Combine. I read something that fills in pretty much all of this.
The Xen universe is like a sort of hub - it is where there are a lot of portals to other universes. I can't quite remember what, but something forced the Synth creatures to flee from their homeworlds to Xen. I think in the first game, the Nihilanth took over the creatues and sent them to Earth. Hence why in the first game, the Vortigaunts were against you, and in HL2, they were free and allied with the humans.
After the Resonance Cascade, the Combine took advantage and took over Xen (I believe this is part way between HL and HL2). Because the Combine were so powerful, they had control over the creatures. They used their technology to enslave the creatures, as is seen in HL2.
As I said, I'll try to find sources to support and confirm this.