I don't use windows or ios bit I recommend thunderbird on windows, k9 mail on android and canary mail on ios. Others are mostly store your emails in their unsecured servers and sell your data
It will always be strange to me why people use email apps on Windows. I haven't willingly used a desktop email program since Yahoo mail was introduced back in the day (then on to Gmail, etc.)
What do people do with email that the web client isn't enough?
(edit) - And just to add, OP check out Thunderbird, Mozilla's (maker of Firefox) email client.
My accounts are all Gmail or Google Workspace. I have always hated the sorting and display options Gmail allows. I've used desktop Outlook forever and have it tweaked exactly as I want it. I'm paying $99 a year for Office360, might as well get my money's worth.
Outlook gave me an opt in to a new client, it's basically the webclient, looks kinda like an webapp. Needless to say it sucks, I hope they won't follow through with this bullshit.
i use 5 different mail addresses at once, some of them are encrypted using PGP, some are not.
I have individual signatures, sometimes several per mail account.
I can preset mails offline, don't have to rely on five different web interfaces and can connect a central address book and calendar to all accounts.
Useful for grabbing and keeping a personal backup of emails. Can be referenced offline if connection to the internet is poor. Native notifications. Better integration into other desktop applications. For Apple users, good syncing across devices.
Obviously web clients could also provide most of these benefits as well.
Thunderbird on Windows, I have used it as far as I can think back (10+ years).
On Android currently FairEmail because the native Gmail app sucks. But it's very much a power user app that's a bit complicated at first, but then works like a charm (It also has a one-time purchase of $5.99 for extra features, no ads, no tracking, no data sent to the app developer).
My external website/server hosting provider uses a pretty much uncustomized RoundCube web interface. this works wonderfully well on computer and mobile.
I just use the outlook web client on Windows, but I use Edison on iOS.
I used Spark for a while but then they made a bit of a crappy design decision on the app. Edison has been a better choice anyway.