• Q1: How do you call it?

A noun preceding and describing other noun is called a noun adjunct or attributive noun or noun premodifier. Source 1 calls it premodifier.

• Q2: Are they still nouns?

Yes, all the sources call them nouns.

• Q3: Are they adjectives?

From source 3:

Nouns used in this way are sometimes said to be adjectives or to behave like adjectives. Attributive nouns may be marked in dictionaries with a label like often attrib placed after the part-of-speech label for noun. While any noun may be used attributively, the label is limited to those quite frequently used in this manner. An adjective is defined as a word standing for the name of an attribute which describes a noun more fully, e.g., "yellow flower."

Source 1 does not write that a noun can function as an adjective.

• Q4: How can a noun function as an adjective (what is the meaning of function)? Is this a rigorous definition?

More research needed

Notes:
-you can also have postmodifiers

-the premodifiers of nouns are usually adjectives, but they can also be nouns, genitive noun phrases, participles, adverbs, numerals, and others.

Sources I found and consulted:

• 1. Oxford English Grammar, S.Greenbaum
• 2. Wikipedia's Noun Adjunct entry.
• 3. Dictionary.com What is the difference between an attributive noun and an adjective?

• Q1: What do you call it?

A noun preceding and describing other noun is called a noun adjunct or attributive noun or noun premodifier. Source 1 calls it premodifier.

• Q2: Are they still nouns?

Yes, all the sources call them nouns.

• Q3: Are they adjectives?

From source 3:

Nouns used in this way are sometimes said to be adjectives or to behave like adjectives. Attributive nouns may be marked in dictionaries with a label like often attrib placed after the part-of-speech label for noun. While any noun may be used attributively, the label is limited to those quite frequently used in this manner. An adjective is defined as a word standing for the name of an attribute which describes a noun more fully, e.g., "yellow flower."

Source 1 does not write that a noun can function as an adjective.

• Q4: How can a noun function as an adjective (what is the meaning of function)? Is this a rigorous definition?

More research needed

Notes:
-you can also have postmodifiers

-the premodifiers of nouns are usually adjectives, but they can also be nouns, genitive noun phrases, participles, adverbs, numerals, and others.

Sources I found and consulted:

• 1. Oxford English Grammar, S.Greenbaum
• 2. Wikipedia's Noun Adjunct entry.
• 3. Dictionary.com What is the difference between an attributive noun and an adjective?

• Q1: How do you call it?

A noun preceding and describing other noun is called a noun adjunct or attributive noun or noun premodifier. Source 1 calls it premodifier.

• Q2: Are they still nouns?

Yes, all the sources call them nouns.

• Q3: Are they adjectives?

From source 3:

Nouns used in this way are sometimes said to be adjectives or to behave like adjectives. Attributive nouns may be marked in dictionaries with a label like often attrib placed after the part-of-speech label for noun. While any noun may be used attributively, the label is limited to those quite frequently used in this manner. An adjective is defined as a word standing for the name of an attribute which describes a noun more fully, e.g., "yellow flower."

Source 1 does not write that a noun can function as an adjective.

• Q4: How can a noun function as an adjective (what is the meaning of function)? Is this a rigorous definition?

More research needed

Notes:
-you can also have postmodifiers

-the premodifiers of nouns are usually adjectives, but they can also be nouns, genitive noun phrases, participles, adverbs, numerals, and others.

Sources I found and consulted:

• 1. Oxford English Grammar, S.Greenbaum
• 2. Wikipedia's Noun Adjunct entry.
• 3. Dictionary.com What is the difference between an attributive noun and an adjective?

• Q1: How do you call it?

A noun preceding and describing other noun is called a noun adjunct or attributive noun or noun premodifier. Source 1 calls it premodifier.

• Q2: Are they still nouns?

Yes, all the sources call them nouns.

• Q3: Are they adjectives?

From source 3:

Nouns used in this way are sometimes said to be adjectives or to behave like adjectives. Attributive nouns may be marked in dictionaries with a label like often attrib placed after the part-of-speech label for noun. While any noun may be used attributively, the label is limited to those quite frequently used in this manner. An adjective is defined as a word standing for the name of an attribute which describes a noun more fully, e.g., "yellow flower."

Source 1 does not write that a noun can function as an adjective.

• Q4: How can a noun function as an adjective (what is the meaning of function)? Is this a rigorous definition?

More research needed

Notes:
-you can also have postmodifiers

-the premodifiers of nouns are usually adjectives, but they can also be nouns, genitive noun phrases, participles, adverbs, numerals, and others.

Sources I found and consulted:

• 1. Oxford English Grammar, S.Greenbaum
• 2. Wikipedia's Noun Adjunct entry.
• 3. Dictionary.com What is the difference between an attributive noun and an adjective?