ICYMI ... Haliey joined Zach onstage at Nissan Stadium, making a cameo during his encore performance of "Revival." Sources with direct knowledge tell TMZ ... HW didn't end up onstage Saturday night just for a singalong, as Zach's team had spoken with people connected to Hailey before the show.

We're told Haliey has always been a major fan of Zach's and bought her ticket for the concert well before she became the viral sensation she is today.

So, as the internet search for Haliey intensified, due to her meme-worthy "Hawk Tuah" quote, the 21-year-old found herself fearing for her safety in crowded public places such as the concert. Remember, prior to the concert, she was laying extremely low.

We're told Haliey alerted members of her newly assembled team about her concerns, and they reached out to Zach's people, who were more than happy to help -- offering Haliey a spot backstage, where she could safely watch the show.

Zach's team also offered Hailey the chance to join him onstage during the encore, a potentially once-in-a-lifetime opportunity she did not let slip.

Haliey and Zach's connection is also how her first-ever interview came about. Remember, Hailey spoke out for the first time on the "PlanBri Uncut" podcast, addressing the multiple rumors about her personal life to host Brianna LaPaglia -- who is Zach's girlfriend.

It's clear Haliey is making the most of her newfound fame, documenting her A-list cameo and interview in her first video upload to Instagram.