Oh joy! I do love ranting, so apologies if I run on. You see, smurfing is just as described, 'a skilled player creating new or low-level accounts to play against less experienced opponents'. This happens all over the place, from highly competitive games like League of Legends and Overwatch, to games aimed at casual matches and more classic experiences like Halo and Call of Duty. However, the systems they use and even genres are completely different. I'm sure you can understand that players get used to a game's quirks, learn valuable skills like decision making, metas (an acronym standing for Most Efficient Tactic Available) and prioritization.
However, a lot of games, including the ones mentioned, understand that it's an issue, and it impacts the most important members of that game's community: the new people, as like any game, without new people sticking around, there won't be anyone to actually have a match with, and this is typically a sign that the game hasn't solved that critical underlying problem for far too long.
Games like Overwatch and League of Legends have a hidden mechanic called 'ELO'. Think of this like a numerical ranking amongst all other players. This is a complicated formula that takes into account multiple things, from your individual performance like Kills, Deaths, and Assists that match to other metrics the game might think is important. In a FPS game, this might include your accuracy, or even compare movements to other players. Games like the above mentioned Dota, this might include gold earned, or how quickly you earned that gold. You may also have heard this referred to as 'skill based matchmaking, or SBMM for short.
Most multiplayer games implement this kind of system, whether they declare that overtly, like a ranking system or they hide it to avoid players abusing the system. You may have heard in the past of developers filing copyrights for such systems, and there is a stigma for what this kind of system can do, as the potential for abuse is very real, but I'll leave that for a different time.
Now, games do try to combat smurfing, as well as other bad behaviors to avoid the worst case scenario of a game dying out, but even games in the same genre do try to innovate on approaches to this strategy. Like I mentioned with ELO, it was a system that tried to identify a good player and 'balance out the teams' so to speak. It was a system, in the purest sense, to balance out matches as evenly as possible.
This is part of the reason why if you've ever talked to someone who's played multiplayer games recently, or watched videos about it, they might refer to games with people 'tryharding' or the lobby being full of 'sweats'. Because every game is balanced, it's not like the game has that natural flow to what would be a minorly imbalanced game, so people have to literally 'try harder', hence the moniker in order to win. As an aside' tryhard' is an old term but been kind of warped in recent times because the old focus was that they were too focused on trying harder to win and not enjoying the game, while today's definition more alludes to people taking a game too seriously, which is ambiguous and has mixed connotations, for me at least.
Regardless, These systems are actually kind of varied. Take Call of Duty's leveling system with new guns and attachments being unlocked as you play. Now, a good player might be able to play really good with just a basic gun given to a starter account, but the different guns and attachments do give a edge to that less experienced player, and if the smurfer does stomp as they intended, they'll level up rapidly and won't be playing against newer players for long.
Other games that make use of the SBMM system I talked about earlier take note of the kind of performance that indicates smurfing, and rapidly pushes that player up the ranks until they suffer losses, ensuring that even if they do play a easy game, it once again, won't be for long. That, 'won't be for long' isn't a common reoccurrence by chance, by the way. These systems are mostly used in what would be competitive games as a whole (not going to get into how lobbies are changing as the people who are in them are either adults now, or new children being shown directly what doors this hobby can open, but it is worth pointing out). This is partially because competitive games need that constant influx of new players to keep popularity surrounding the game and interest generated for the competitive leagues that these companies try to generate, as it presents a massive revenue stream if it works out, as Overwatch and League of Legends have shown directly.