So, once you've confirmed that your version of IIDX uses IFS files, you're gonna need a way to open those IFS files. Lucky for us, the lovely mon already did all the hard work for us and made a program called IFSTools.
Download it here by clicking the latest version under the "Releases" text on the right side of the page

and then clicking "ifstools.exe" under "Assets"

The readme on the Github page explains pretty damn well how to use IFSTools, but we're not gonna need every single one of those commands, so I'll give a quick rundown on how to use IFSTools.

The quickest and easiest way to extract your IFS files is just by dragging and dropping the IFS file into ifstools.exe, like so:

There's a chance that Windows will ask if you're sure that you want to run the program, just click "Run"

A terminal window will open and it's showing you what files it's currently extracting.

Once it's done extracting everything, you should be left with a folder in the IFS's directory, that uses the same name as the IFS file you selected.

You can also extract IFS files by opening IFSTools in a cmd/terminal window. By putting the exe's path in a terminal window, followed by the path to an IFS file, it will extract the IFS file, and put the extracted folder into your user folder (C:/Users/username).

Or, if you use "-o" after your IFS file, you can set the output directory.

Inside your newly extracted IFS folder, there will be a few subfolders.

The one we're interested in is "tex" - that's where textures are stored. All actual textures in the tex folder should just be saved as PNGs, so we don't need to worry about extracting more shit from them :)

Now that we understand how IFS files are extracted, here's a super quick guide to which IFS files do what:

Files that begin with...
"beam" are keybeams
"exp" are note explosions
"fcombo" are full combo animations
"gmframe" are background frames, NOT play frames. just the background that holds the BGAs, pacemakers, play info, shit like that.
"judge" are judgefonts
"lane" are lanecovers
"notes" are noteskins
"qp" are QP items
"turn" are turntable stickers
"gmframe" are play frames (these may not be in the graphic folder, but in data>graphic>0 or data>graphic>1)

Then, there's other files that may be of interest, such as
"gameparts.ifs" which holds things like the HI-SPEED sliders, text such as "1st Stage" and "Extra Stage", and the Effect indicators that show at the bottom of the screen.
"common.ifs" which holds things like "Clear" "Failed" "Hard Clear" "Ex-Hard Cear" text, "Normal" "Hyper" "Another" "Leggendaria" text, and generally used textures.
Or "logo.ifs" which holds the splash screen logos and warnings.

Don't be afraid to dig around and really get your hands dirty in the files! There's a LOT of files to go through, you never know what you may find!