I can't find a thread more appropriate for this question: How does one build a balance team? Not specifically for OU, but generally. Like, what's the make up, and how do balance teams play?
Balance teams have a ton of versatility, so this question is challenging to answer without going wayyyyy too long for one post to explain. However, the goal of balance teams should be to combine offensive presences and Pokemon that can make progress in games with defensive presences that are capable of checking/countering opposing Pokemon and stopping/minimizing the progress they may be able to make. You should also make sure to have a certain degree of focus on synergy, meaning that your Pokemon should work well together and work together to achieve progress in games or victory as an end result. For example, the core of Heatran and Tangrowth works well as a balanced backbone seeing as Heatran is capable of setting up Stealth Rock, trapping things like Clefable or Toxapex, and doing offensive damage while Tangrowth is able to check problematic Ground or Water types (i.e: Greninja and Zygarde) while also crippling/chipping away things with various attacks such as Knock Off, but it is more passive against bulkier presences like Clefable or Toxapex.
My best advice for balanced teambuilding would be to try and make sure that you have what you personally feel is a sufficient offensive gameplan to beat bulkier teams while also having enough of a defensive backbone to check/counter common offensive threats. It is really a middleground playstyle that is intended to maximize consistency, so try not to overemphasize on any individual strategy or element, but rather cover as much as you can both offensively and defensively without compromising anything fundamentally necessary to the teams ability to function.
Here is an example of a balance team I made for a teammate of mine to use in a tour a couple months back. As you can see, it has a defensive backbone with Celesteela, Tangrowth, and Mega Tyranitar; these three synergize well with each other while also not being overly passive, thus promoting cohesive gameplanning and gameplay that does not leave you consistently falling behind against faster paced teams. On the other hand, Tapu Lele, Ash Greninja, and Tornadus-Therian provide an offensive side to the team, giving us the ability to break through other teams as the game goes on, set up Spikes, remove opposing hazards that may impede upon our own play, and even generate our own momentum. Not all balances need to follow any specific blueprint or even look anything close to this as it is a very versatile playstyle that can use a plethora of different viable Pokemon, but just make sure to try to generally cover what I touched on above and you should be taking steps in the right direction!
