I realise that Google places ads in the native Gmail environment. However, when using a third party front end to embed the Google services, how does the company make money off that? When I use Thunderbird to access Gmail or the Google Calendar or when I map my Google Drive to the file explorer, how can Google profit from this kind of use?
None of those are business though, they are non profits
Protonmail (and other proton services) has a Free Tier, and it’s company, Proton AG, were For Profit for a long time (until recently).
Tutanota (now known as Tuta) also has a Free Tier, and is still, to this day, a For Profit company.
Bitwarden allow you to store unlimited passwords, they are a For Profit compant.
Standard Notes let you have Unlimited Notes, and was For Profit until they got bought by Proton
Notesnook also allows Unlimited Notes, and is For Profit.