For director Mike Mills, filmmaking lately is all about family.

His new movie, “20th Century Women,” set in 1979 and centering on three female characters and their influence on a 15-year-old boy, was chiefly inspired by Mills’s relationship with his mother, with bits and pieces of the two younger women drawn from his older sisters. This isn’t the first time the 50-year-old filmmaker has used family as fodder for storytelling. His 2011 “Beginners” — which earned Christopher Plummer an Oscar for his portrayal of a man who comes out as gay in his 70s — was based on Mills’s father.

We spoke with the California-based director recently by phone about mining his personal life for artistic purposes.

Q: “20th Century Women” centers on your 15-year-old alter ego, Jamie (Lucas Jade Zumann), and his relationships with several characters: his 55 year-old mother, Dorothea (Annette Bening), a draftsperson and amateur pilot; his 17-year-old platonic crush, Julie (Elle Fanning); and two of his mother’s boarders, a 24-year-old photographer named Abbie (Greta Gerwig) and William (Billy Crudup), a handyman in his 40s who is sleeping with Abbie and flirting with Dorothea. How do you diagram the film’s ambiguous relational dynamics?

A: For me, the Jamie-Dorothea relationship is the sun of this solar system. It has been called a coming-of-age film, but if so, it’s a really bad coming-of-age film. The boy never changes. If anything, Dorothea does. I kind of enjoy it being purposely ambiguous, because it is about how these people figure themselves out in relationship to one another. The weird thing that happened in the writing and the editing was: OK, if Dorothea is narrating the part about Jamie, who is the protagonist of that moment? When we were showing the film to people, we’d get completely different answers. I was, like, “Huh, that’s interesting.” I kind of enjoy that, actually.

Q: Is there an overarching theme in your work about emotional transition — letting go of one thing and embracing another?

A: I never would have put it that way, but I like the way you just phrased that. To me, all my films are about people trying to figure out who they really are. Not who they’re supposed to be, not who they’re told to be. It’s done in the context of the intersection between family and nonfamily, whether a romantic or other kind of relationship. I guess that’s my turf.

Q: The title of the new film suggests that, if the film is a coming-of-age tale, it’s one about the coming of age of the modern woman. Is that accurate?

A: Absolutely. With my mom being born in the 1920s, that’s really quite an expanse there. I was born in 1966. My mom was born in 1925. I’m a punk-rock, skateboarding kid in Santa Barbara in the 1970s. My mom was like Amelia Earhart and Humphrey Bogart put together. She’s from another world. She doesn’t look right, doesn’t talk right. I’m always having to explain the world to her, and explain her to the world. That’s another seed of the movie. Other people help you figure yourself out. That’s really true of myself, my sisters, my mom, my first girlfriend. These are the people who helped me get over some very limiting, normative ideas of the world and myself. “Writing a love letter to my mom” sounds kind of grandiose. It’s more like a very long thank-you note.

Q: In the film, your mother’s house is a sprawling, old structure under constant renovation. I get the feeling that it’s a metaphor for something that will never be finished.

A: Everything is transitory, even these epiphanies we have. In my movie, toward the end, Jamie and Dorothea reach a level of honesty or connection with each other that she’s always been craving but that’s not going to stay forever. The movie is trying to appreciate the transitory nature of everything. That house — you’re so right — that’s so my mom, and an extension of her romanticism. I grew up in a house kind of like that.

Q: Jimmy Carter’s “Crisis of Confidence” speech is a touchstone in the film. How did you come to use it?

A: Carter was very popular with my mom because he wore blue jeans in the White House. In searching for which year to set the film in, I wasn’t sure: ’77, ’78 or ’79? When I found out that the speech was in the summer of ’79 and I read it, I was, like, “Oh, my God.” The speech is both so prescient to now — to the undermining of things that Americans typically put faith in — but also so completely impossible for a politician to say now. It’s too vulnerable. It’s filled with too much doubt. It’s about how we are feeling a lack of meaning in our lives. I just had this hunch that I could transpose this speech by a U.S. president — a very macro-level speech — to the inner lives of my characters. That toggling back and forth between the personal and the political is what I really love. When I can do that, I’m so excited as a writer.

Q: Another signature of yours is the use of montage: archival images and vintage objects that define a time and its people. What do some of them signify?

A: A lot of the objects are real. My mom read “Watership Down,” and that wood rabbit that’s in the movie, my mom carved that. She wore Birkenstocks and smoked Salems. Annette’s wearing my mom’s jewelry. That beautiful bedspread that she’s lying down on is my parents’ bedspread. There are paintings from my house in the movie. I do believe you’ve got to sort of enchant the world you’re making, and these objects hold meaning. ... I love it when other filmmakers put personal things in their movies. It just has a certain grippiness when you put in these unexplained chunks of your life.

Q: In that sense, doesn’t the character of Abbie — who creates a self-portrait by taking Polaroids of all her possessions — have a bit of your DNA, too?

A: You’re right, I’ve done it. I’m fascinated by it. The film is itself partially the product of self-portraiture through not just you, but the text around you. By “text,” I mean music, objects, cultural moments and other people, in a way.