You can find them in the premium_cache folder, and under skins of your Minecraft storage directory, as above. However, some skins introduce custom models and higher texture resolution which cannot be transferred over to Minecraft Java Edition. There is this workaround (scroll up or down, depending on your sort settings) that allows you to access your skin, if it is a standard model skin.
iOS (you can access a sandboxed version of this via iTunes Sync), but the actual file location is: /var/mobile/Containers/Shared/AppGroup/ or /Apps//Documents/games/com.mojang/ (depending on whether you are using sanboxed or rooted/jailbroken).Windows 10, this is: %localappdata%\Packages\Microsoft.MinecraftUWP_8wekyb3d8bbwe\LocalState\games\com.mojang\.
If you used an imported skin, you can find them in \minecraftpe\custom.png of your internal storage. In Bedrock, unlike Java Edition, skins are a little messy there's several types of skins available Regular Skins
