How do companies usually handle that?
They don't - it's a part of your compensation that is variable. It's a risk you take taking part of your compensation in equity. But, the risk goes the other way, too - if the stock performs well, they don't reduce your compensation to "match the market".
Also note that "the market" in the US is highly variable based on location, and since non-executive salaries are private, there's no way to accurately measure what the "market" rate is for any given position.
If you feel that your compensation is "below market" or there's too much risk in the equity portion, you can ask for a change in allocation, or an increase with whatever protocol your company allows (it may be as simple as a conversation with your manager). Or look around for a similar job that pays more and either take it or use it as negotiating leverage.
I don't know German culture, but in the US, compensation (especially for for professionals) is highly individualized. Companies may use (or say they use) some formula to determine salary, but in the end they'll pay you whatever you'll accept to work there. I've managed teams with members that varied in salary by as much as 75%. The percentage increases were generally consistent based on performance, but there was no automatic process to make sure people were "at market". If you were paid less than market and either didn't know or didn't want to negotiate, then that's where you stayed.
--- EDIT based on comments ---
If the RSU is just a "sign-on" bonus that is clearly stated in the offer, then treat it as such - make sure you are comfortable with the ongoing salary alone.
If it's part of a periodic "bonus" program, then see if it says whether the equity portion is fixed or variable.
I've worked at places where your annual "bonus" was paid out in part cash and part equity, with the equity portion adjusted based on the current stock price. So if the stock plummeted, your cash portion of the bonus was set but you'd get more shares (e.g. you could expect to get $20k in cash and $20k in equity, getting however shares it took to get to that amount).
But also remember that bonuses are not always guaranteed and can be cut depending on company performance.
In short, it again depends on whatever you're willing to accept. If they don't offer enough or there's too much risk in the equity/bonus portion, then look to see if you can get a better offer elsewhere.
Maybe this scenario is one of the reasons for the short retention of employees in US tech companies?
I think there's a lot more to "job hopping" than having a variable portion of compensation.