Fixed it! Learned that the order of the custom editors matters. Thank you for your help!

Tried doing this, now it just opens it up in Notepad++ even though I've set .zip and .gz to use 7zFM.exe

srnyx wrote:

I’ve tried setting it in the editors page but it still used file explorer.

You need to "Edit" the file, not "Open" for the settings to take effect. You can make the "Edit" be the default double-click action (what it actually it):
https://winscp.net/eng/docs/ui_pref_panels#double-click

I’ve tried setting it in the editors page but it still used file explorer.
My default is file explorer but I don’t wanna change that cause I only use 7-zip to edit files in archives (which is what I wanna do in WinSCP).

Do you mean that you want WinSCP to open double-clicked file in 7-Zip? Is 7-Zip the default application for .zip files on your machine? How do .zip files open in Windows File explorer? If you to use 7-Zip with .zip files, change the default .zip application in Windows Settings. I you want WinSCP to behave differently than Windows File Explorer, you can change it in preferences:
https://winscp.net/eng/docs/ui_pref_editor

I want WinSCP to use 7-Zip when opening a ZIP file instead of using File Explorer.