Learn Croatian: Djevojka s tetovažom pije sok s ledom. - The girl with a tattoo is drinking juice with ice.

Both are examples of the preposition s (with) followed by the instrumental case, which is what you are learning now! However, they show two slightly different uses: s tetovažom describes a physical attribute of the girl, while s ledom describes the contents or additions to the juice.

Why do both tetovaža (feminine) and led (masculine) get the exact same -om ending here?

The preposition s requires the instrumental case. For most feminine nouns ending in -a (like tetovaža), the -a changes to -om. Most masculine nouns (like led) also take the -om ending in the instrumental case (unless they end in a soft consonant, which takes -em). They are different noun genders that just happen to share the same ending here!

Why does the sentence use the short form s instead of sa?

You only use the longer form sa when the next word starts with a hissing sound that makes s hard to pronounce (like s, z, š, or ž). Because tetovaža starts with a t and led starts with an l, the short form s is easy to say and correct.