Sorry for the delay - I had to sleep on this one!
If the folders are on your iPhone, they should be in webmail (AFAIK). When signed into webmail - what browser are you using? Make sure you aren't zoomed in - that can affect the options you see on some pages.
Personally created folders show down below the automatic folders - look for MY FOLDERS. You need to click on the arrow to get them to show. This picture has the blue arrow pointing at Archive and covering up My Folders label, but you can get the idea!
Now as to why they might not show in Outlook - I don't use Outlook, but I have seen advice about subscribing to folders - search the help text or internet for that setting. (maybe this: https://support.microsoft.com/en-us/office/show-or-hide-imap-folders-b8474acf-e8e1-4d1a-be33-708a1ea...)

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