Learn English – In person Vs personally

If you're frightened of dogs, I would recommend you say that and not say "I'm not a dog person." Saying that you're not a dog person implies that you just dislike dogs for some reason. A dog owner might try to change your feelings by getting you to engage with their dog, because they can't imagine anyone disliking the pet that they find so appealing. If you say "I'm scared of dogs.", most dog owners will understand that is not something that you can easily change your mind about, even if their pet is a really nice dog.

Here's another example: if my friend and I are talking about food and I say "I'm not a shrimp person.", my friend might say "Oh that's just because you've never tasted MY shrimp recipe that everyone says is really good! I'll make some for you tomorrow." If I had said "I'm allergic to shrimp.", my friend might say "Oh that's too bad - I have a really great recipe for shrimp. Maybe I'll try it with chicken next time so you can taste it."

When you tell someone that you don't like something that they themselves like, it's just human nature to try to persuade you over to their point of view.

Addressing your specific questions:

'Can you imagine how dirty it gets over time?'

When people talk informally, they are often a little sloppy. The present tense would be the perfect choice to state a generalization or natural law such as:

'White gets dirty over time.'

This idea was close enough for the speaker. Someone who likes to be more precise might say, instead, "Can you imagine how dirty it would get over time [if we bought the white pillow]?" But you might not enjoy living with someone who uses English precisely all the time. Such people tend to be nitpicky.

'The grease from your fingers will burn into the bulb and then it breaks.'

Here, two ideas have been spliced together into one sentence.

First idea: "[If you touch the bulb with your fingers] the grease from your fingers will burn into the bulb."

Second idea: "When that happens, i.e. when you touch the bulb with your fingers, the bulb breaks."

This is another example of the simple present being used for a generalization or natural law. And again, in a situation of informal speech you caught the speaker being a little sloppy.

'I hope it goes away overnight.'

I don't know how the English grammar experts would view this (if you want to know, you could ask over at ELU SE), but I will share how I see this example. My other primary language is Spanish, which has a subjunctive. English has one too, but people don't think about it much. In Spanish you really can't get away without thinking about it. In Spanish, this would be

Espero que se quite para mañana. ("se quite" is conjugated in the subjunctive)

I see the English sentence the same way -- I see goes away as the present subjunctive, which happens to be conjugated the same as the simple present.

Your questions were good, and you've done some careful listening and recording of what you've heard.

I would be remiss if I didn't point out a small but important misuse of the simple present tense in your question:

You wrote: I moved from Germany to California and since I'm here I hear people use the simple present....

Expressions beginning with "since" are notoriously easy to get tripped up on.

Better: I moved from Germany to California and since I've been here I've heard people use the simple present....

Or: I'm from Germany. Here in California I hear people use the simple present....