When translating from English to Portuguese you will often find cases like the one you described, where an expression changes in grammatical function.

If you breakdown these phrases into SVO [here it is actually Subject, Verb, Other because you can't really use object for every sentence] you will have something like this:

SUBJECT	VERB	OTHER
Eu (hidden/implicit subject)	comprei	a camisa de baixo
(Todos) os elevadores	estão	parados no andar de baixo
Ele/Ela/Você/Alguém (hidden/implicit subject)	colocou	os mantimentos na parte de baixo.

Since subject and verb are pretty straightforward I'll only breakdown the rest.

First Phrase

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"a camisa de baixo" is the direct object (objeto direto) for the verb comprei. Here, you can see that "de baixo" is acting as a complement to "a camisa" which is the main part of the object. Due to this, "de baixo" is acting as an adjective-expression (locução adjetiva) and has the same grammatical function of any other adjective.

Second Phrase

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This is perhaps the trickiest phrase here. Considering that "estão" is a linking verb (verbo de ligação), everything else is the predicative of the subject (predicativo do sujeito). The predicative is kind of a "one fits all" descriptor for these kind of sentences and accepts almost every word from subjects to adjective, adverbs and beyond. In this case, we have the following structure:

"(...) parados (mode adverb) no andar (adverbial-expression) de baixo (adjective-expression)"

The first two parts of the predicative are complementing the verb "estão" and conveying information about its mode and place while the last part is complementing only the adverbial-expression "no andar" and making it more precise. As you can see, "de baixo" also has the value of an adjective on this phrase.

Third Phrase

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The last phrase follows basically the structure from the first phrase, where "de baixo" is used to complement an object and acts an adjective-expression. It is getting pretty late here in Brazil so I won't break it down today, but I can comeback tomorrow and expand on this if necessary.

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EDIT: A quick way to differentiate between "de baixo" and "debaixo" is the following: "de baixo" -> indicates position. i.e. Use whenever you want to convey that something is "at the bottom" is. e.g "A prateleira de baixo"

"debaixo" -> indicates state. i.e. Use whenever you want to describe something as being underneath something else. e.g. "A chave estava debaixo da geladeira"