Why are the Eldian's in possession of Warships?
If you're talking about the Eldians outside of Paradis Island (the one's where [Spoiler Alert] Reiner come from), that was because of the Marleyans. Due to the Marleyan's fear of the Eldian's titan transformers, they created weapons, technology capable of destroying these Titan transformers. That's how the Marleyans got into possession of four of the original nine titan transformers (wont explain it here).

Sources: Manga. It was mentioned by one of the Marleyan higher ups. Forgot the chapter. But besides that, it was heavily implied.