Hello Robander!
Let's see... Borrowings are words adopted by the speakers of one language from a different language. It is a consequence of cultural contact between two language communities. For example, English speakers adopted the word
garage from French, or
perfume, at first with a pronunciation nearer to the French pronunciation than is now usually found. Presumably the very first speakers who used the word in English knew at least some French and heard the word used by French speakers.
The very same thing happens in spanish, in this case with this particular word you mention "birra". Those who first use the new word might use it at first only with speakers of the source language who know the word, but at some point they come to use the word with those to whom the word was not previously known. To these speakers the word may sound 'foreign'. However, in time more speakers can become familiar with a new foreign word. The community of users can grow to the point where even people who know little or nothing of the source language understand, and even use the novel word themselves. The new word becomes conventionalized. At this point we call it a borrowing.
Conventionalization is a gradual process in which a word progressively permeates a larger and larger speech community. As part of its becoming more familiar to more people, with conventionalization a newly borrowed word gradually adopts sound and other characteristics of the borrowing language. In time, people in the borrowing community do not perceive the word as a loanword at all. Generally, the longer a borrowed word has been in the language, and the more frequently it is used, the more it resembles the native words of the language.
It's a very interesting topic concerning languages! Hope it helps!