Alright, going to offer some critiques since you asked for them.

Disclaimer: I'm trying to make objective criticisms but some of what I say may be my own stylistic biases speaking.

In general, the problem that pops out most to me is how poorly your ideas flow into each other, mainly because of poorly-structured sentences and paragraphs. I know this is a vague, so let me give some examples.

>Shooting upright, he believed
>Scanning his surroundings, he was perched atop

Here you use a sentence structure which goes "[A]ing [B], [C] [D]ed", in which A and D are verbs, B is an adverb, and C is the subject performing both A and D.

When using this structure, "[C] [D]ed" is supposed to be something that directly follows from "[A]ing [B]", not only chronologicaly, but also logically or causally.

"Moving quickly, he pounced." "Looking back, they saw." "Shrugging it off, she spoke." These are all examples of this structure in which the second action logically follows from the first.

In your sentences, "he believed" does not logically follow from "Shooting upright", nor does "he was perched" follow from "Scanning his surroundings". This could be fixed by changing the second actions to something that does follow from the first. For instance, "Shooting upright, he looked around," and "Scanning his surroundings, he saw that he was perched"

Another approach that would work for the first case would be to switch the position of the actions within the structure of the sentence. "Believing it was an earthquake, he shot upright," or "He shot upright, believing it was an earthquake," both work because shooting upright does logically follow from thinking there's an earthquake going on.

(Incidentally, I would have used "thought" instead of "believed" in this sentence. Thinking is a more immediate, short-term action, while believing usually involves more careful consideration of the available information, so thinking works better as a term for a snap judgement like this.)

>Tapping the home button, it stuttered to life

Here you used the same sentence structure again. This time the two actions do follow from each other, but there is another problem: [A] and [D] are both supposed to be actions which are carried out by [C], but in this sentence they're carried out by two different subjects. You have Tom tapping the home button, and then "it" (the phone) stuttering to life, but because of your misuse of this structure, the implication is that the phone is performing both actions. It would work better as something like "Tom tapped the home button and the phone stuttered to life."

>Another brief quake. Were they getting stronger? He was still wearing his sweatpants from the day before with his running shoes and loose fitting jacket with a long sleeve shirt below.

Here the first two sentences follow from each other, but the third doesn't follow from either of the previous ones. In fact, it marks a complete break from the line of thought of the other two sentences because the clothes he's wearing don't have anything to do with the earthquakes

If you want these exact sentences to be in this exact order, you should at least put a line break between the second and third one to make the first two sentences their own little paragraph. The third sentence should be the start of a new paragraph.

>He brought his hand to mouth to chew on his knuckle. “Did I even make it home last night?” The sky was still dark but heavily obscured by the foliage overhead.

Same problem here. The last sentence starts a completely different line of thought from the one that came before, so it should also be the start of a new paragraph.

>The coarse surface left deep creases in the wooden structure beneath him.

Unless I'm missing something, the coarse surface and the wooden structure are one and the same thing (the giant root), so you're essentially saying that the coarse surface left deep creases on itself, but the creases were already there, they weren't left there by anything. It should simply say "Deep creases ran through the [coarse surface/wooden structure (choose one)] beneath him."

There's a bunch of other sentences or paragraphs I could point to, but I think that's enough examples for now. Basically, you just need to learn how to better structure your thoughts into sentences and paragraphs.

Another general observation is you could really stand to trim down your wordcount by a lot. It would really help make your writing more impactful, I think. Taking the first sentence as an example, and without doing anything too fancy to it, I would have written it as "The ground shook violently below Tom." Besides making it more concise, "The ground shook" is also a stronger start to a story than "Tom felt". "Tom felt" jumped out to me as a really weak opening from the moment I read it.

For one more quick example, you wrote "Tom silently hoped", but "silently" is unnecessary since hoping is already a mental, and thus silent, activity.

Regarding the story itself, I didn't see much to complain about. There's just one thing that felt off as I was reading this first chapter, which is when the giantess said, "I swear you are by far one of the most interesting humans I have ever met." It just doesn't feel like Tom did enough to be seen as all that interesting.

Anyways, that's all I care to say for now. Hope this doesn't discourage you and you go on to improve your writing.