How to change a Minecraft Bedrock Resource Pack, Into a Java Resource Pack

Setting Up

Java resource packs have a different structure entirely. First, I recommend you get 7-Zip and go to %appdata% and then .minecraft > versions > x.y.z and get an uncompressed folder. Then delete all the classes, meta, com and net garbage and just leave the assets folder. Inside that folder you can find everything. Now you are left with the equivalent of downloading the resource pack for bedrock from minecraft.net for Java Edition. You can see the directories and where everything should be. Note a few things are switched, for example the destroying textures are in blocks/ on Java, whereas on Bedrock they are in environment/.

Naming Conventions

Bedrock

Bedrock tends to have the descriptor after the object for naming conventions. Take the following examples: wool_blue, mutton_raw. This is very useful because it means that all of something are grouped together in the file explorer. (i.e. all the wools start with wool_ so they are put right next to each other).

Minecraft 1.8

Let’s take an older version, for example 1.8. The naming convention is pretty much the same as Bedrock.

Minecraft 1.16

The names here more reflect their in-game unlocalized names. Like blue_wool. This is a lot harder because now all the blues are grouped together, but all the wools scattered about. Get ready for a lot of meticulous renaming as like half of the textures have their name just flipped! Super frustrating and a big task.

Manifest

In Java there is a MCMETA that manifests the info. Look on this Gamepedia page:

{
 "pack": {
   "pack_format": 6,
   "description": "Tutorial Resource Pack"
 }
}

This should be in the same directory as the assets folder. (Not inside the assets folder, beside it!) It should be named pack.mcmeta.

Pack Format Guide

Different versions require different pack_format values. They are as follows:

1 in 1.6.1 – 1.8.9
2 in 1.9 – 1.10.2
3 in 1.11 – 1.12.2
4 in 1.13 – 1.14.4
5 in 1.15 – 1.16.1
6 in 1.16.2 – 1.16.5
7 in 1.17+

Where To Put It When Done

Go to %appdata% and then .minecraft > resourcepacks and put the folder in there.

Distribution

Just zip the folder (should be folder that the assets folder is inside of). Have the other people also put the zipped folder in their resourcepacks folders on their computer and then have them unzip it. Make sure you don’t distribute Minecraft’s original textures! Delete the textures if you didn’t change them, as any texture without an image will go to the default texture anyway. While realistically no one would really care if some random dude gave his friend a folder with some Minecraft textures its still illegal!

Sounds

A little bit of extra work, try the Gamepedia Article for info.

Not much of an answer, given you marked your answer as correct already, but I want to add on here. Once you've zipped your resource pack folder, rename the entire file from ".zip" to ".mcpack". This makes the import process far simpler as Mojang's pack system literally is just a bunch of renamed zip files. The extension does nothing more than tell the game what type of pack it is. double click the .mcpack, and the game will automatically import everything, assuming you have the manifest set up properly.