An outside power brings them together.
My favorite way of bringing the group together is to have them all summoned by the initial quest giver. When I'm being lazy or ham-handed, I'll even require that they include some way that they have a favorable relationship with the first quest giver in their character background.
The quest giver can then help provide an opportunity for them all to introduce themselves and then provide the opportunity for the quest.
This is particularly easy in games like Shadowrun where the characters are usually mercenaries or mercenary like. The first event in the game proper can be meeting a potential Johnson. In something like D&D, an official might put out a call for help, and the official might then set the meeting for all of them at the same time. Its a little harder in something like Exalted where the characters are often not mercenaries, but even then if everyone is required to have a tie to someone specific, they can be expected to assemble if that person puts out a call for help and they'll get to know each other along the way.
Similarly, an NPC can go around assembling a team, not unlike what Nick Fury did in the MCU, and meet the characters separately before bringing them together. This is basically the same idea, except with the quest giver doing the travelling instead of summoning.
On the flip side, they can all start in trouble together. They can be prisoners that need to start by escaping for instance. While not exactly this, the opening to Divinity Original Sin 2 was along these lines.
This can be handwaved.
I suppose this is technically something of a frame-challenge, but it is worth noting that this is very often simply handwaved. They are a team because they are a team and no further thought on that topic is needed.
In the animated version of Vox Machina, they literally start in a tavern, but they are very plainly already a team. At least thus far, very little information on how they met or why they became a team is ever given. Explanations for how they individually became adventurers are provided but not until much later, and short shrift has been given to how they met or formed a team.
This is very common, particularly in games like DND. They can be declared to be a team by fiat and nothing further needs to ever be mentioned.
Some people that are highly invested in the story might find that unsatisfying, but if you have a group of adventurers that at least at first will be doing classic adventurer stuff and particularly if you plan to focus on exploration and combat, not having too much of a backstory for how they became a team is perfectly fine.
The slightly softer version of this is that they assemble as a team during Session 0 and just mutually write it into their backstories. This works very well if there is cooperation and coordination during character creation (which has other benefits outside the scope of this question). It is still essentially by fiat, but it does provide a background story.
As Phoenix Duck already said, the classics are fine
Also, I agree with Phoenix Duck. While a bit cliche, "You all meet in a tavern" is a perfectly fine way to start an adventure. Tropes are tools. Don't be afraid to use them, particularly while you are new and figuring things out.