Wise is the pop star who knows how to preserve a little mystery about themselves, even in the age of “over-sharing” and the microscope that is social media.

Her most obsessed fans can find out bits of this and that. Some, I dare say, even figured out where she lives with her family in a middle-class home in Greater Los Angeles (Highland Park).

But there’s so much we don’t know, and still won’t know after watching the intimate and somewhat revealing documentary profile, “Billie Eilish: The World’s a Little Blurry,” on Apple TV+.

We meet a seemingly-unguarded, generally-unfiltered and determinedly unselfconscious young woman, tracked through her quick rise to fame late in her 16th year, through her 17th and well into her 18th.

Director R.J. Cutler, who did the Vogue doc “The September Issue,” the superb supernatural teen death drama “If I Stay” and the TV doc series “American High,” was well-suited to the subject. But as we see the teen use her mom as a stand-in and home props to block out her concept for a music video, as we see her sketch out songs (basically storyboards), the visual and emotional effect she is going for with them, as we hear her bluntly tell her mother how she’s going to warn this or that music video director about what she WON’T be doing, we can wonder how much control Cutler had over the project.

I mean, nobody is identified in it. We can figure out who her dad is, her mom and Finneas the brother/collaborator/co-star. None are ID’d by name. So this person giving feedback is from…her management company? Interscope Records? That one going over her physical regimen — her free-form and athletic dancing gives her shinsplints and “throws out” her neck at one point — is a physical therapist, personal chiropractor, trainer, chakra choreographer?

Chelsea?

She’s 19, stupidly famous and ridiculously open-hearted around her fans. Mosh pits and hugs for people standing in line don’t see very Queen Bey or Tay Tay. So if she wants to keep her location, her family’s names and the somewhat-overwhelmed and handsome African American boyfriend she had at the time of filming out of the public eye, that’s understandable.

She calls him “Q” and he has a $700 haircut and bright red dye job, and that’s enough.

But 140 minutes of “fly on the wall” filmmaking, mostly of Eilish performing, songwriting (or avoiding it, as her brother complains), doing photo shoots and radio interviews and peaking at Coachella is a bit much. Celebrating her staggering sales and Grammy success with a family-gifted Dodge Challenger (We watch her pass her driving test, and give her car its first wash) and recycling her Creation Myth — social media star, bedroom recording her debut LP “and she’s so young” –leaves the film a “blur.”

Glimpses and only glimpses of home movies show the full-throated childhood of Billie Eilish Pirate Baird O’Connell and the “musical family” she grew up in.

“Our family was just one big f—–g song!”

Clips show her in the dance training she underwent until injuries curtailed that. She mentions her body dysmorphia without connecting that to dance, but declares her aversion to “tiny, tight outfits.” As unselfconscious as she seems, she is super-self conscious about that, thus the oversized shirts and baggy shorts that are her uniform.

She confesses to her years of self-injury, cutting herself as punishment for whatever failings she saw in herself at 14. She mentions having Tourette’s, but she comes off like any other unfiltered teen you run into these days.

Eilish likes drawing genitals and dropping the F-bomb, and the only hint of ego slips in with a “How do people NOT miss me? I would miss me!”

Her art comes from “never feeling happy,” but she’s pretty giddy a lot of the time. Who wouldn’t be? But “I feel the dark things,” obvious from her music, is what sells. Her brother is the one with his eyes on the business here, Mr. “We need to write a hit” in this R-rated version of The Carpenters. Getting her to sit down and collaborate on a tune is a chore, though. And she’s already complaining that this or that new material is “just like everything else we’ve ever done.”

She gets it. If the distinct and once red-hot Miley and Adele and Katy and Lady Gaga can turn a tad passe, Eilish won’t be able to avoid it.

Childhood Justin Bieber fan reluctant to do a duet with her now-married longtime skirt-chasing idol, radio interview guest who reminds us of how young she is by all the pop culture references she doesn’t get, and Queen of Coachella who gets a backstage visit from the four-model-years previous queen, Katy Perry.

“This is gonna be WILD for 10 years,” Perry warns her. “So if you wanna talk…”

No, the 17 year-old girl with “Pirate” in her name and a mania for “Pirates of the Caribbean” doesn’t recognize the handsome man Katy introduces as her fiance, Orlando Bloom. But once she makes the connection, she reaches out for a second meeting like every fangirl in her audience.

The tour stops and performances pad “A Little Blurry” to its bloated 2:20 running time, not that fans will mind. As someone who wondered if she had real singing chops when she whispery processed voice first blew up, I was impressed with a couple of performances. But like her, you can hear the songs are “just like everything else we’ve ever done,” and that makes the film drag and drag in its last hour.

She’s just 19. Eighty minutes would have been more than adequate to cover her life thus far, especially if you aren’t interviewing her or even identifying everybody in that life with her.

MPA Rating: R, profanity and lots of it.

Cast: Billie Eilish, Finneas, Maggie Baird, Patrick O’Connell, Katy Perry, Orlando Bloom

Credits: Directed by R.J. Cutler. An Apple TV+ release.

Running time: 2:21