Learn Farsi: یک لایه‌ی خزه داخل تراریوم گذاشته‌ام. - I have put a layer of moss inside the terrarium.

The word لایه (layer) ends in the unpronounced 'h' (written as ه, pronounced 'eh'). When you link it to the next word with an ezafe to say "layer of moss", you need a consonant bridge between the two vowels. That bridge is ی, making it لایه‌ی خزه (laaye-ye khaze).

Is there an ezafe between داخل (daakhel) and تراریوم (teraariyom)?

Yes, even though it is not written out. Prepositions and location words like داخل (inside) connect to their target noun using an ezafe. You pronounce the phrase as daakhel-e teraariyom.

How is the verb گذاشته‌ام (gozaashte-am) formed here?

This is the present perfect tense, meaning "I have put". It is built by taking the past participle of گذاشتن (to put), which is گذاشته (gozaashte), and attaching the "I" ending ام (am).

Why don't we see the indefinite 'i' suffix on the noun phrase to mean "a layer"?

While you often see the suffix ی added to the end of a noun phrase to mean "a" or "an" (like لایه‌ای), you can also just use یک (yek) at the beginning of the phrase. Here, یک لایه‌ی خزه works perfectly to mean "a layer of moss".