Although Biel, 39, and Timberlake, 40, welcomed baby Phineas last July -- and didn't confirm the birth until January 2021 -- Biel said they didn't intend to keep everything under wraps.
"I had, like, a secret Covid baby," she told Shepard. "It wasn't like it [was] supposed to be a secret. It was just Covid happened and then I went to Montana with my family and never left."
"The Sinner" star went on to detail her experience giving birth during the pandemic, noting that Covid safety restrictions in the hospital "had just changed" so Timberlake was able to be present for the delivery.
"There was a moment there that there was nobody allowed at all and I was really getting nervous about that situation," Biel explained. "But yes, he was allowed. I think if I had to be there alone, that would have been horrible. I would have been really scared."
The "7th Heaven" actress -- who also shares son Silas, 6, with Timberlake -- also opened up about her life at home with her family.
"I'm spending a lot of time at home on Zooms in my apartment," Biel said. "Hanging with my baby, changing diapers, doing nap time."
"Someone said to me, 'Two is like having a thousand.' And that's exactly what it feels like," she later added. "The balance of everything is very different and super hard."
"I mean, it's amazing. It's so interesting. It's so funny," Biel explained. "The conversations I'm having now with my 6-year-old is so cool. Like, he's a real person saying the funniest stuff and he's so sensitive and tender. It's just so interesting to see that part of it happen and the little one is just cute as hell."
The "Stealth" star also shared her thoughts on her sons possibly working in show business someday, admitting she has mixed feelings.
"My knee-jerk reaction is 'Oh god, no. Please no,'" she said. "But then I look at these kids and I'm like, 'Oh s---- they're probably going to be musical. What are we going to do? Like, not let them play the piano or not let them take a voice lesson if that's their passion?'...I don't want to be that parent to stifle a dream. But man, if my kid would just be like, 'Let's go learn about corn in Iowa.' [I'd be like], 'Great.' I would so much rather them be an engineer or something."
The "Valentine's Day" actress also discussed her career, revealing that her agents wanted her to like Angelina Jolie back in the day.
"Being a woman [in Hollywood] you're always looking at like Meryl [Streep]," she shared. "That's the career that you just do anything you want and it goes forever and she's the classiest, coolest. [Also,] Glenn Close or even Julia Roberts at that time."
"Can I tell you who I would have picked for you just looking at you? Angelina Jolie," Shepard said, to which Biel replied, "I think my representatives ... that was sort of the goal. I was sort of feeling Glenn Close and they were going, 'Let's do 'Tomb Raider' and 'A-Team,' shoot some guns and wear tank tops.'"
The "Total Recall" star said that trajectory "worked for me for a while."
"That totally was my jam for a while," Biel admitted. "Those were cool experiences. [But] I feel like I look back at my slate...and I go, 'I don't know who that is. Who is that person?'"
"It was cool, doing all the different things in one's career happens for a reason so you can't regret anything," she continued. "But there was a lot of times where I was kind of on this Angelina Jolie kind of path and it just wasn't creatively fulfilling."
Shepard pointed out that it's not fair how female actors only have had a "few options" when it comes to career routes, while men are "encouraged to just be" themselves.
"It's really true," Biel said in reply. "It's obviously getting better. It's gotten a lot better for women. Television and streaming networks and platforms with the different opportunities, but women are still dealing with all the same b-------."
"We're still dealing with the physicality of being a woman," she added. "Do you use that? Do you not use that? Are you being encouraged? It's confusing to try to know how to handle the way, historically, women have been categorized and put in these boxes."