Learn Polish: Nie mam YouTube'a w telefonie. - I do not have YouTube on my phone.

This directly relates to the lesson on foreign loanwords. In Polish, when a foreign proper noun (like a brand or app) ends in a silent 'e' in its original spelling but ends in a consonant sound when spoken, you must use an apostrophe before adding Polish case endings. Since YouTube is pronounced ending in a /b/ sound, we attach the -a ending with an apostrophe to preserve the original spelling.

Since nie mam requires the genitive case, is -a the genitive ending here?

Yes! The negated verb nie mam forces the genitive case. However, even in a positive sentence (I have YouTube), you would still say mam YouTube'a. In modern Polish, tech brands, apps, and social media platforms are usually treated as masculine animate nouns, so their accusative form matches their genitive form.

How is the word YouTube'a pronounced in this sentence?

Despite the apostrophe, you pronounce it smoothly as a single word: roughly /ju-tu-ba/ or /ju-tju-ba/. The apostrophe is strictly a spelling rule to handle the silent English 'e'. It does not represent a pause or a catch in your throat when speaking.

Why does Polish use w telefonie (in the phone) instead of na telefonie (on the phone)?

When talking about software, apps, or files stored on a device, Polish uses the preposition w (in) with the locative case. The device is viewed as a container holding the data. You generally use na (on) for physical surfaces, like na ekranie (on the screen), though na telefonie is sometimes used informally for doing an active task on your phone.