I learned most of my creature and entity modding by taking the vanilla humans1 and looking through the creature tags page of the wiki and adding the ones I wanted my race to have while keeping an eye out for tags I didn't want them to have and making sure they were removed. Then after adding and removing a few tags, running a test, making sure things worked as desired, then added more.
Testing was just a matter of duplicating the dwarf entity and putting the creature in it and changing the entity id. Figured out what tags worked, which tags didn't, and kept a close eye on the errorlog for tags that I used wrong. Then modified as necessary. Once I got the race more or less acting as desired I moved on to the entity raws. Doing the same thing as I did with the creature, adding and removing tags as seemed sensible.
Nowdays you can use the arena test mode for most of the creature testing, without needing to create a test entity. but you'll wanna do some live world testing as well since not all problems show up in arena mode. For example the [CARNIVORE] tag appears to work just fine in the arena, but when placed in worldgen the critters are unable to obtain enough food to expand or sometimes even survive.
And yes, if you have the opportunity starting with reactions is a good way to learn the raw structure. But you also learn best when you are motivated and inspired, so there's no need to start there if you can't think of any reactions you actually want to make.
1I used humans, but you want to start with a creature as close as possible to your desired race. Only exception I can think of is animalmen. Those use fancy shenanigans in vanilla, use humans or a similar modded critter as a base for them instead.