If your husband has a Fold and it works for him you should be able to use the same setup. So the question is "what is different?", which unfortunately isn't easy to work out remotely.
In general all of the relevant settings should be in one of 2 places: the settings for the calendar app (which I guess is Outlook from your comments) and the settings for the Outlook account (which you should be able to find in the "accounts" section of the phone's settings - if there isn't one then it sounds like the Outlook app is as restricted as it was last time I used it).
Unfortunately I don't use either the Outlook app or a Samsung phone (Samsung change the menus to their own style) so it's hard for me to guess details. I would look at the calendar app's settings, find the settings for events and see which account is set as the default for creating new events. My guess is that that is set to the Yahoo account, in which case setting it to the Outlook account may be all that you need (or specifying the calendar each time you create a new event, but it's better to have a sensible default).
You can sync other calendar apps with Outlook if you install an app that sets up the Outlook account as a general calendar account rather than something specific to the Outlook app (sorry, can't think of a clear way to say what I mean, except that this is what I meant when I blamed the Outlook app if you don't have an Outlook account visible in your settings). MS used to have an app called Outlook.com, and if you synced that with your outlook account you could view and edit the calendar in almost any other calendar app as well. Then they replaced that with the Outlook app and suddenly the calendar wasn't available to any other apps - which is when I ditched that app. I actually use a paid third-party app called Nine to manage my MS accounts, and with that installed I can manage the calendars in any of my other calendar apps as well. But if your husband has a working setup it should be possible to replicate that instead.
Anyway good luck. Changes of platform can take time. Personally I find Android's approach to settings, where you access an app's settings from the app itself, more logical as it's closer to how my computers work (even my Apple computers), but familiarity is the biggest part of things like this. And I always warn people making any change of platform or even OS version, you'll tend to notice the things that don't work the way you expect before you come to understand anything that works better, so initial impressions tend to be negative no matter what change you make.