Yes and No.
A round represents about 6 seconds in the game world. During a round, each participant in a battle takes a turn. (PHB 189)
The only thing wrong with the above answer is their definition of a turn, A turn is literally mere moments when compared to the 6 second time-frame of a round. This is counted as a specific ruling, technically. A round is about 6 seconds, meaning that a give or take of 1 second is allowed. So even though a ROUND is 5-7 seconds long, turns are different. A round is defined as a 5-7 second time-frame in which the full order of initiative is acted out until it resets to the highest initiative. You are right in your statement about the approximate simultaneous actions within a round, with the rolled initiative standing as each character's grasp of the situation and their reflexes at that combat instance.
Though no matter that a Round consists of 5-6 seconds, it can be comprised of thousands of Turns if it needs. Just look at massive scale wars. Basically a turn in this mindset is just the character's moment in that 6 seconds to take their actions.
Now on to the Sneak attack part:
Beginning at 1st level, you know how to strike subtly
and exploit a foe’s distraction. Once per turn, you can
deal an extra 1d6 damage to one creature you hit with
an attack if you have advantage on the attack roll. The
attack must use a finesse or a ranged weapon.
Let's break this down now.
Once per turn,
To be precise, this allows the Rogue in question to use their Sneak Attack feature at any turn, not just their own. Nice
you can deal an extra 1d6 damage to one creature you hit with an attack if you have advantage on the attack roll. The attack must use a finesse or a ranged weapon.
Now this is where the RAW meat is, lol. The effect activates at ANY time your weapon would deal damage, NOT just when you take the Attack Action (which is usually limited to use on your own turn). Here's an example:
If a feature let's this rogue use an attack as a Bonus Action outside their own turn, and the environment fits with the rest of the Sneak Attack description (have advantage, or not needed when a foe of the target is within 5 feet of the target, isn't incapacitate, and you don't have disadvantage), you get to add your Sneak Attack damage the damage inflicted.
Let's say this rogue and his ranger buddy team up on a single Orc (poor orcs are always abused). The ranger is at one end of the hallway while the rogue sits in an alcove out of site of the target Orc. The rogue is effectively hidden because of full cover, the orc might know there is something there but can not discern the exact location. The rogue then uses Cunning Action to Hide as a Bonus action. The ranger can then fire at the Orc to get it's attention and the Orc charges the ranger. The second the Orc attempts to leave the space right beside the hidden rogue, since almost all rogues will roll a higher Stealth than an Orc's passive perception, they can use their Sneak Attack along side their Reaction to get an Attack of Opportunity on the passing Orc.
There might be more instances of using an attack outside of your own turn, but without a book directly in front of me (only my character sheet), I can not give every example of how a sneak attack can be used.