Turns out, it's actually possible to download custom packs on Xbox One without any external devices! This guide will cover every step of the process. There aren't many visual aids so you'll need to be content with descriptions of your tasks. If you would like a video guide instead, watch this video by dakonblackrose: https://youtu.be/MFKO1HdwTlE

First and foremost, navigate to Settings > Display & sound > Video output > Video fidelity & overscan and uncheck the "Apps can add a border" box.

After that's done, you'll need to download two apps from the Xbox Store: File Download and UFO Transfer. As unprofessional as these look, they're key in installing custom worlds and packs. The icons look like this:

Now that we're all set up, let's download the pack. Open Microsoft Edge and navigate to the download screen for your pack. This method may not work for all sites, but Mediafire works just fine. Hover the cursor over the download button, but instead of downloading, press the Start (three lines) button and choose Copy link.

Hop on over to File Downloader for the next step. Select the three lines in the upper left corner and navigate to ┌> From URL. Click on the Source URL box and scroll up until your selection leaves the keyboard and becomes a cursor. Hover over the Source URL box, press the Start button, and choose Paste. Click the arrow under the right end of the box to restore the screen to pre-keyboard state. Next, select the Local Storage folder and press the arrow under the right end of the Source URL box again. Finally, press the start panel to begin the download.

Select the three lines in the upper left corner again, this time navigating to Storage. In here, you should see multiple folders. Choose Local Storage, and now you should see your downloaded pack. Type the name of the pack (or anything else, but the name is for organization) in the box that says Local Storage above it. Once the name is in the box, click the three dots in the bottom right of the screen and choose "new folder" from the list that appears. Press UP to return to the previous menu with your pack.

If your pack is in a ZIP file, skip to the next paragraph. If not, select the checkbox next to your file, click the three dots in the bottom right corner again, and pick "rename". Change ".mcpack" to ".zip".

Click on your ZIP file and you should see the contents of the pack you downloaded. Press the button with two checkmarks and an unchecked box in the bottom right corner of the menu. This should select all the files on screen. Click on the button in the bottom right with two squares and an arrow to copy all the files. Now press UP to go up a folder and select the folder you created. Once you're in the folder, click on the clipboard in the bottom right to extract your resource pack into the folder. This could take a little while, so be patient.

For the next step, we need to open UFO Transfer. Once it loads, select the gear in the upper right corner and then choose "Open in File Explorer". That's all we needed UFO Transfer for. Seriously!

Now that you're in the File Explorer menu, navigate to the top of the screen and press "Packages". Scroll through this list of folders until you find "Microsoft.MinecraftUWPConsole..." and select it. Navigate to "LocalState > games > com.mojang". Create a new folder by selecting the button in the bottom right hand corner with a folder and a + sign. Name this folder exactly "resource_packs" without the quotes. Create another folder here called "behavior_packs", again without the quotes.

Navigate back to the Packages menu and this time look for "25974MegaScopes.Universal...". This is where you can access files from File Downloader. Select it and then select LocalState. Here you should see your pack folder! Choose the button on the bottom right with two checkmarks and four lines and check the resource pack folder. Click the scissors icon to cut (copy but remove the source afterward) your pack.

Navigate back to "Packages > Microsoft.MinecraftUWPConsole... > LocalState > games > com.mojang" and choose either behavior_packs or resource_packs depending what you downloaded. Finally, choose the clipboard icon in the bottom right corner to paste the pack folder here.

You're finally done! All that's left is to make sure it works in-game.

An important note is that the pack *will not appear* in the Global Resources tab. It's only accessible from the Resource Packs and Behavior Packs tabs when editing a world. From there, you can add the pack and rejoice that this lengthy process granted you the ability to use whatever you downloaded on Xbox One!