In English, we communicate with reliance on letters, words, phonemes, syllables, intonation, etc. ASL has a similar structure that goes something like this:

30% of all meaning in a signed discourse is found in the signer's facial expression.
25% is found in body movement
20% is found in classifiers (English doesn't have too many)
20% is found in proper signs (what most people think of when they see "signing")
5% is found in fingerspellling

Deaf people rely on fingerspelling for most proper nouns (unless the community already has an established sign, like a family member's name, local or national cities, etc.). They may use fingerspelling to convey an exact turn of phrase or concept , which would be more like "lexical borrowing" or borrowing from a foreign language.