Learn English – can “today” be used for present, past and future

It does not matter whether it has ever flown before or it's only flying one time.

The simple present can be used for something that is happening for sure (irrespective of the number of times it has happened in the past or will happen in the future).

See these -

The movie starts in 10 minutes - Talking about the movie which is starting in a few minutes. It could be for a movie that has never been shown before i.e. this is the first show. Nor am I talking about the future shows of the movie.

In the same way,

The train departs in 10 minutes, hurry - I am concerned about the event that is happening for sure in the future.

So, you can say that whether the flight happens to be for the first time or for the last time.

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....