Learn Hardcore German: Der Mechaniker arbeitet in einer kleinen Garage am Ende des Weges. - The mechanic works in a small garage at the end of the path.

Because Der Mechaniker is the subject of the sentence – the one performing the action. In German, subjects always take the nominative case, and der is the masculine singular nominative definite article.

Why is the phrase in einer kleinen Garage in the dative case?

The preposition in is a two-way (Wechselpräposition) preposition. When it indicates location (no motion into), it requires the dative. Garage is feminine, so the indefinite article becomes einer (dative feminine) and the adjective klein takes the mixed-weak dative ending -en, giving kleinen.

Why does kleinen end with -en instead of -e?

In the dative case for feminine nouns with an article (definite or indefinite), adjectives always get the ending -en. This is part of the standard declension pattern: dative adjectives take -en regardless of gender when an article is present.

Why is it am Ende des Weges and not im Ende der Weg?

am is the contraction of an dem, using the dative because an here indicates location.
Ende then takes a genitive complement to express “of the way.” The noun Weg is masculine, singular, and in genitive gets -es, so des Weges.

Could I say am Ende vom Weg instead of am Ende des Weges?

Yes. vom is the contraction of von dem (dative), and in colloquial or less formal German am Ende vom Weg is common. However, am Ende des Weges (genitive) is more formal and appears more often in writing.

Can I move am Ende des Weges to the front of the sentence?

Absolutely. You can start with the time/place phrase for emphasis, but you must keep the finite verb in second position:
“Am Ende des Weges arbeitet der Mechaniker in einer kleinen Garage.” The meaning stays the same.