I've got a 3-line plan with UL data with 2 iPhones and one dumbphone on the plan, each with UL data (AT&T Unlimited Plus). They call what I have a "shared data usage" plan, although each device is subject to the relevant data caps for usage and MHS. I didn't have to set this up, it was available to me from day one.
The "Settings>Usage" section shows a single-page breakdown for the current billing cycle. On that scrollable page there's tabs for data/text/talk, followed by a pie chart showing all lines as a percentage of usage, followed by data usage by number. In the pie chart field, at the bottom, is an optional "How did I use my data?" link, described below:
The disabled-by-default option is to enable/disable a breakdown for each line showing browsing and technology, other, news, and social - those options can be selected to show a further breakdown. I use a couple of VPN and data logging apps - that data shows up as "technology" when I tap on the "+" next to "browsing and technology".
There's also a link to the account's full site but I find navigating that web site a PITA on an iPhone screen. I hope this helps.
