A cross was done among plant with the desired trait to create the breeding population. The breeding population was usually phenotypically selected for desirable traits.
That phrase is confused. To phenotypically select from a breeding population, you first have to generate the breeding population, which requires an initial cross of desired plants. The generations are generally labelled as F1 F2 F3 and checked for statistics. They can back-cross the mother plant onto herself. The selection refines itself as the number of crosses of the most desirable plants increases in generations.
Hybrid populations are often stronger and have a wider gene base, so if you want big fruit for a tree, you can cross an asian one with a european one, it might be more resistant and vigorous, gain stronger roots, better low N performance, and better structre from one plant, which is then mixed with the desirable traits for example of yield or mould resistance. The wider the gene base, the more potential diversity of individuals can be chosen which also contain the alleles from the desired traits, and the selections start with more options. The goal is to maximize the desired pheno's.