US movements offer clear examples. In 1965, Filipino farmworkers led by Larry Itliong began the Delano grape strike and asked Cesar Chavez's mostly Mexican American union to join. The groups merged in 1966 into what became the United Farm Workers. In 1969, the Chicago Black Panther Party under Fred Hampton formed the Rainbow Coalition with the Young Lords and the Young Patriots, a group of poor white migrants from Appalachia.
Solidarity is different from charity. Charity flows one way and leaves power where it is. Solidarity treats the struggle as shared, which is why it is closely tied to mutual aid and labor organizing. Labor unions use the word to mean that workers support one another's strikes even when they are not directly affected.
Solidarity can be hard because groups face racism in different forms, and divisions are often encouraged by those in power. Building it usually means following the lead of the people most affected and showing up consistently, not only in moments of crisis.