As I understand it, MrBeast helped fund the creation of ViewStats [0] in order to gather more data on thumbnails and channel / video performance over time. Then this knowledge is applied to their own content in order to make it even more successful. At this point there's probably multiple people who specialize just in thumbnail optimization.
Another key detail about MrBeast production is that they target a global audience, so they hire famous voice actors of every major language to do their voice-overs. A few years before YouTube supported multiple audio tracks, they had different channels for various languages and regions. Now it's just a drop-down in the video settings. Many products fail to take internationalization and localization seriously, so their products are unable to penetrate non-western markets.
Speaking of international reach, I saw in an interview a few years back that MrBeast was trying to expand to the Chinese market, but none of his public interviews since then have discussed how he's doing there. This goes a bit against the extreme focus on YouTube as his primary platform. A quick search on bilibili (which I believe is the Chinese equivalent of YouTube), shows his latest video hitting 1.6 million views and 8k comments, which isn't bad but it doesn't really compare to the amount of attention that he gets on YouTube. It seems like even the most skilled content creators in the West still struggle to break into the Chinese market.