Learn Portuguese (Portugal): Ela compra roupa bonita.

In Portuguese, roupa is commonly used as a singular collective noun to mean clothes or clothing in general. While the plural roupas does exist (meaning specific outfits or pieces of clothing), the singular is much more natural for everyday conversation about clothes.

Since this lesson is about reflexive verbs, how would I say She buys herself beautiful clothes?

To make this reflexive and show that she is doing the action for herself, you use the third-person reflexive pronoun se. In European Portuguese, this pronoun typically attaches to the end of the verb with a hyphen, giving you: Ela compra-se roupa bonita.

Why does bonita come after roupa instead of before it?

In Portuguese, descriptive adjectives like bonita (beautiful) generally come immediately after the noun they describe. Notice that it also ends in -a to agree with the feminine singular noun roupa.

Why isn't there an article like a or uma before roupa?

Just like in English, you do not need an article when talking about an unspecified quantity of something in general. She isn't buying the clothes or a specific item, but rather just clothes in a general sense, so no article is needed.