How to understand allusions in poetry?

In "Short Songs", the author used the following expression techniques:

1. Reference.

Because the famous sentence Zheng Feng Ji Zi in The Book of Songs shows women's deep yearning for their lovers and the poet's desire for talents; Quote the sentences in The Book of Songs, Xiaoya Luming Literature, which describe the banquet of guests, and express the poet's expectation and courtesy for talented people.

2. metaphor.

The bright moon is a metaphor for talents, the bright moon is a metaphor for the scarcity of talents, and the black magpie is a metaphor for wandering on the wrong path of talents, expressing concern about their future, and "the mountains are never too high, and the sea is never too deep" is a metaphor for its broad mind to accept talents from all over the world.

3. Use allusions.

Borrowing Zhou Gong's allusions, he expressed his sincere treatment of the sages and hoped that the sages in the world would come back and create a good situation of "returning to the world".