What is "Safari services" listed in iOS cellular data usage?
According to Apple, Safari Services is a framework that application developers can utilize to integrate Safari behaviors and features into their applications. For example, when you open a link in Discord (a messaging app), it opens in a mini version of Safari, from which you can quickly return to your chat, or open the link in the 'full version' of Safari.

You might be seeing excessive data usage if you frequently open data-heavy websites like YouTube through other apps like Twitter, for example. I should note, however, that I tested this on my phone, running iOS 14.5 (18E5199a) and could not find the mentioned &quot;Safari Services&quot; category; instead, I see the relevant data usage attributed to Safari itself. It's possible that there has been a recent change regarding this behavior.