Very relatable content as I have a 2.5 year old and an 8 month old at home. Different things work for different kids, when my son was an infant and would start crying for seemingly no reason, the fallback was always to pat his mouth with my open hand so his cries turned into a wahwahwahwahwah noise. It almost always turned into laughter. My 8 month old daughter unfortunately does not care for this tactic. She either wants to be held in some obscure fashion, for example she likes her arms over my right shoulder so she can look behind me and pat on my back while I bounce her legs up and down a bit, or she wants me to lay down so she can crawl/climb on me. She probably picks that last one up from my son who always wants to wrestle around, he's her favorite.

Now that my son is a toddler, he's fairly decent at saying what's wrong but the things that bug him are far more diverse, so it's really just a game of getting him to calm down and helping him explain why he's upset. He got mad at me the other day because I was cleaning our back yard and I moved a stick. Apparently he was "putendenden" (pretending) that it was a lion or something. I probably asked him about 20 questions about the lion and why it was in our yard and where it came from. That calmed him down, he wanted to have fun pretending and was afraid I was ruining it, by asking questions I validated that I wasn't encroaching on his ability to have fun. We snuck up on the "lion" and surprised it, I looked like an idiot to any neighbors that might have been watching, a good time was had by all. That story came to mind when I read tip 10 on this post.

The fun part about toddlers is you can proactively teach your them how to do things for themselves right up until the point they get frustrated (and a hair past it), which helps them not only learn to be self-sufficient in practice but also to be mentally resilient to that feeling of frustration. I've noticed a difference in how often we get total meltdowns from our toddler since my wife and I have put in extra effort doing this. It helps right up until close to bed time, apparently being tired disrupts a toddlers ability to be resilient to frustration :)