It's mostly just the fear of immigrants in general and populist right wing stuff like "they take out jobs", "they will replace us" and such. It's general xenophobia against all kinds of people, be they eastern European, from the Balkans or Mediterranean.

Ironically, studies have shown time and time again that the areas with the highest concentration of AfD-voters are also the ones with the least amount of immigration. Adding to that, the AfD has a very large social media presence and there are a ton of people spreading fear, propaganda and, to a large part, Russian disinformation. They always make the most outrageous claims and offer the most simple solutions that have no basis in reality. Some people are unhappy with rising prices and stagnating economy and are blaming it on the "new" government that hasn't had a chance to clean up after 16 years of conservative Christian rule and was elected in the middle of multiple global crises.

It's the good old fOrEiGnErS bAd narrative. They phrase it as "illegal" migration, though, because phrasing it more bluntly would be illegal in Germany for historical reasons.

The fear of all kind of non white immigration. like all far right parties they arw strongest in areas with low or no immigration though

All immigration, but if they say that out loud people will call them fascists and there's nothing that makes a fascist more upset than getting called a fascist before they think they can largely ignore any social consequences which normally would come with that distinction.

From what I can tell, not actual immigration but irrational fear of immigration and also a good portion of racism is causing support for AfD.

It’s the immigration of people with darker skin color that Germans don’t like. Nobody ever talks about EU-immigrants. Or Ukrainian refugees. The media don’t mention them, the politicians don’t propose laws to give them a hard time here. It’s all about African, Afghan, Syrian, and even the quite well integrated people with Turkish roots.

I think the answers others have given you definitely apply to the very far right voters. They are against all form of immigration. But that doesn't explain the AFD's rise in popularity. So I'm going to talk about the more moderate rightwing people here. There is a lot of immigration into Germany, EU migrants and legal, as well as illigal immigration from outside the EU. Plus Ukrainian refugees. Most people on the right don't mind EU migrants as well as Ukrainian refugees. Ukrainian refugees btw. are mostly women and children. Legal migrants from outside the EU are more complicated, and I'm sure there are various levels of acceptance. The real problem though are illegal immigrants, mostly from the middle east and Africa. They are mostly men with poor education and in many areas the infrastructure to accommodate them are at their limits. It doesn't help that these places are neglected and violence does break out at some of them every now and then. These stories obviously spread on social media like wildfire.