One of the most important resources for anyone making a resource/texture pack, is to have a copy of the default resources— for the version you are working on. This lets you see the correct folder structure, and what all the files should be named. Mojang likes to rename a bunch of files about every other version, so you can’t rely on old information remaining true.
If you are working on Bedrock check out this link: https://www.minecraft.net/en-us/addons/ Scroll down to the bottom for a small link for “resource packs”. You’re done!
The default textures are a harder to get from Java, but the game has already downloaded every verision of minecraft you have played. If you want the resources for a new version of minecraft, make sure you start it up, to make the launcher download it.
For Os X you need to go to Library/Application Support/Minecraft/Versions/ If you can't see the Library folder it's probably hidden. "Shift" + "Command" + "." will reveal the hidden files and folders in whatever finder window you currently have active.
For Windows you need to go to .minecraft/versions/
There you should see a lot of folders. Choose the folder named with the version of minecraft you want.
Inside will be two files. You want the .JAR file.
Using your .ZIP opening application of choice, open it up. If your .ZIP app doesn’t want to open it, you may need to make a copy of the file, and replace the .JAR with .ZIP.
Inside will be a ton of cryptically names files. Don’t worrry, you only need one thing— the “assets” folder. There you will find all the default resource.
When assembling your resource pack, assets, goes directly inside the folder that names your resource pack. So for me it’s Lithos-Core/assets/ and all the folders under that will mirror the folder structure in the .JAR.