My 3 year old IPhone SE claims that it is full, but I don't have much on it? Details?

Over half of the storage is being taken up by "other." I cleared out my safari cache, and I don't have a reading list. I have 35 apps including the ones that come with the phone. I've off loaded 12 of them. I have like 5 pictures. I automatically delete texts after a month. I only have 55 contacts. E-mail and What's App are the 2 things that I can't clear out, but What's App only takes up 1.26 GB, and my e-mail only takes up 557.9 MB

Added (1). How do I get rid of the other? If I reset to factory settings, do I lose my apps, contacts, texts, e-mail, etc?

Does Apple Watch have a monthly bill?

I want to get the Apple Watch series 3 with GPS + Cellular, but will it cost more every month since it's connected to my iPhone I already have? Like I don't want to pay 2 phone bills.

IPhone lightning headphones issue?

I recently bought a pair of Bluetooth headphones. They're no wireless. But they connect through Bluetooth and won't play otherwise. The issue is, they work fine at home but the minute I go outside and off my front doorstep. They start to beep really loudly and then the volume goes all the way down but the music doesn't Stop playing. If I turn the music back up it goes straight back down. I don't understand the issue. Other headphones don't do this. Is it a settings problem?