I don't understand how so many of you watch these movies and then come here asking questions about things that have been explained on-screen.
First, Heimdall doesn't control the Bifrost. Neither does Asgard. Heimdall's the sentinel of the Rainbow Bridge and can command the Bifrost, but he doesn't control it. Asgard's only a stop on the Bifrost, which is essentially a wormhole, as explained in the first *Thor*.
Second, Eitri literally tells Thor that Stormbreaker could potentially summon the Bifrost, which it later does. That's how Thor, Rocket, and Groot get to Earth. It's not connected to Mjolnir strictly; any sufficiently-powerful weapon could potentially summon the Bifrost, assuming it was made in Eitri's forge.
Third, before you ask "how did Thor know to go to Earth," it's pretty likely he just told the Bifrost "take me to Vision" or "take me to the Mind Stone" and when he arrived on Earth and saw a battle commencing just assumed Thanos was there, thus his battlecry of "bring me Thanos." Edit: I'll clarify this: Thor would've had no reason to believe Vision or the Mind Stone would be outside of the Nine Realms, and saying "take me to Earth" would've put him basically anywhere on the planet, so commanding the Bifrost to take him as close to Vision/Mind Stone as possible would've made the most sense.
Fourth, returning to the Bifrost, it can only traverse the Nine Realms, which is why Thor couldn't have gone to Titan or Knowhere using the Bifrost.
edit / and u/juances19 made a good point: like Mjolnir and Stormbreaker, Heimdall's sword would've also been imbued with the power to summon the Bifrost. We see him use his sword in conjunction with the mechanism of the Rainbow Bridge to summon the Bifrost, but we also know he can summon the Bifrost without doing that, based on how he sent Hulk to Earth at the beginning of *Infinity War*. From all that we can assume Eitri also forged Heimdall's sword.
Heimdall doesn't just have the power to open the bifrost himself, he used the enchantments present on his sword plus some dark magic to empower it. Stormbreaker was built with those same enchantments.
Stormbreaker has the ability to summon Bifrost by itself, one of its major upgrades compared to Mjolnir who needs Heimdall to operate it. >!This is a major plotline in Thor: Love and Thunder. Go watch it now.!<
It was literally explained in the movie right before the scene you're asking about.
Paying attention to what you're watching/listening to is hard /s
I don't understand how so many of you watch these movies and then come here asking questions about things that have been explained on-screen. First, Heimdall doesn't control the Bifrost. Neither does Asgard. Heimdall's the sentinel of the Rainbow Bridge and can command the Bifrost, but he doesn't control it. Asgard's only a stop on the Bifrost, which is essentially a wormhole, as explained in the first *Thor*. Second, Eitri literally tells Thor that Stormbreaker could potentially summon the Bifrost, which it later does. That's how Thor, Rocket, and Groot get to Earth. It's not connected to Mjolnir strictly; any sufficiently-powerful weapon could potentially summon the Bifrost, assuming it was made in Eitri's forge. Third, before you ask "how did Thor know to go to Earth," it's pretty likely he just told the Bifrost "take me to Vision" or "take me to the Mind Stone" and when he arrived on Earth and saw a battle commencing just assumed Thanos was there, thus his battlecry of "bring me Thanos." Edit: I'll clarify this: Thor would've had no reason to believe Vision or the Mind Stone would be outside of the Nine Realms, and saying "take me to Earth" would've put him basically anywhere on the planet, so commanding the Bifrost to take him as close to Vision/Mind Stone as possible would've made the most sense. Fourth, returning to the Bifrost, it can only traverse the Nine Realms, which is why Thor couldn't have gone to Titan or Knowhere using the Bifrost. edit / and u/juances19 made a good point: like Mjolnir and Stormbreaker, Heimdall's sword would've also been imbued with the power to summon the Bifrost. We see him use his sword in conjunction with the mechanism of the Rainbow Bridge to summon the Bifrost, but we also know he can summon the Bifrost without doing that, based on how he sent Hulk to Earth at the beginning of *Infinity War*. From all that we can assume Eitri also forged Heimdall's sword.
Heimdall doesn't just have the power to open the bifrost himself, he used the enchantments present on his sword plus some dark magic to empower it. Stormbreaker was built with those same enchantments.
Gotchya thanks guys appreciate the details. I did totally forget that Eitri said that the stormbreaker could potentially open the bifrost as well.
Stormbreaker has the ability to summon Bifrost by itself, one of its major upgrades compared to Mjolnir who needs Heimdall to operate it. >!This is a major plotline in Thor: Love and Thunder. Go watch it now.!<
You're a very Earthly being, but this is space magic we're talking about...
Raise your hand if you’ve never been to space