The Uerdingen line is supposed to separate regions where people say "ik" instead of "ich".
Does this apply only when people are speaking their local dialect, or do people north of the Uerdingen line say "ik" even when speaking Standard German?
That does not reflect current spoken standard German (anymore). Most people speak Standard German and will pronounce 'ich' as [ɪç] also North of the line (at least in my experience, and I grew up North of it and live again North of it). Some people will use the dialect form, though, especially in the Berlin region 'ik' is quite common.
There is a much newer survey which shows the pronouncation of 'ich' within German speaking regions in the "Atlas der Deutschen Alltagssprache"; [ɪç] is predominant in most parts and the other versions found much less, however especially in the South 'i' [ɪ] or [i] is the most-used pronouncation.
A standard language is a standardized form of a language. Standardization means, that there are well defined rules for grammar, pronunciation and all other aspects of a language. These rules are taught in schools, and these rules are applied when official documents (like laws and other legal documents) are written.
So, be definition a standard language is something that belongs to a political entity, usually a country. And from this definition also follows, that a standard language is uniform in the whole region where it is used.
So, Standard German is an attempt to overcome all isoglosses and create a uniform language without regional differences. And this makes Standard German an artificial language that everywhere is different from the local dialect, and (almost) everywhere different form the regional and social colloquial speech.
This means, that in theory by definition isoglosses like Uerdinger line, the very similar Benrath line or the Speyer line can't exist in any standard language.
As just said, Standard German is an artificial language that by some individuals has been created and defined over a period of many centuries to create a common language that is legible by anyone who speaks a German language like Bavarian, Franconian, Allemannic, Saxonian, Frisian and many others.
So, when people try to speak Standard German in different regions, their speech is sometimes closer to Standard German and sometimes farther away from it. And this is the reason, why you can hear the influence of dialects and colloquial speech also in situations when people use Standard German.
Another important fact to know is, that there is not one Standard German. I said before »... a standard language is something that belongs to a political entity, usually a country.« And this is true for German too.
There are three major countries where German is the main language. These are:
Besides them there are some other countries where German is an official language, but the number of German speakers in these countries is to small to define extra standards for them, so they use one of the three standard mentioned above:
The differences are enough to fill many thick books, but what they have in common is much much more. Differences are:
Orthography is unified across all three German variations. Different spellings are very rare and have their cause in different pronunciations only. (Example: German »Geschoss« is Austrian »Geschoß«)
All books printed in German language are printed in German Standard German (with very few exceptions). Even books published in Austria and Switzerland are printed in German Standard German because people in Germany are not used to other standards than their own, but they are 85% of the potential buyers of books.
People in countries where Austrian or Swiss Standard German is the standard are fluent in their own standard and in German Standard German (at least passively, so they can understand anything that is written or spoken in German Standard German). But most people in Germany are completely unaware, that there are other standards than their own. Most of them think that people in Austria and Switzerland speak some dialect all the time.