Learn Turkish: Çocuk internetten biraz müzik indirir. - The child downloads some music from the internet.

The verb root here is indir-. The rule for the aorist tense is that verb roots with more than one syllable ending in a consonant take the -Ir suffix (following 4-way vowel harmony). Since the last vowel in indir is i, the suffix becomes -ir, giving us indirir.

Why is the suffix on internet spelled -ten instead of -dan or -den?

We need the ablative case (meaning "from") to say "from the internet". Usually, this is -dan or -den. However, because internet ends in the hard consonant t, the suffix must also start with a hard consonant to match it. This changes the d to a t, making it -ten.

Does the verb indirmek only mean "to download"?

No, its literal meaning is "to bring down" or "to lower" (it comes from the root inmek, meaning "to go down"). In the context of computers and the internet, it translates perfectly to "downloading" a file!

Why doesn't the word müzik take the accusative case here (like müziği)?

We only use the accusative case for specific or definite direct objects (like saying "the music"). Here, the child is downloading "some music" (biraz müzik) in general. Because it is indefinite, müzik stays in its bare dictionary form.