Now, let’s take a look at these two pages.
For some effects, sometimes it helps to say them out loud. But also this one’s a bit special because it uses ブ, which can be a “bu” sound or a “vu” sound. I’d go with “vroom” as the sound effect here. (Coincidentally, the English release of the manga agrees.)
By the way, if you want to type the っ Belthazar mentioned, you can probably get it by typing “ltu” or “xtu”.
This one’s ガ, which looks similar to が except missing a line and with straight lines rather than curves…カタカナ.
Typically (not always) a single letter or word not in a word balloon is a sound effect. Here, it’s a sound effect for him grabbing Yotsuba’s head.
I don’t know if it’s quite patting, but it is indeed the sound of the action.
Welcome to the fun world of different fonts. Her response begins with 「うあー」 (an exclamation).