In order for a strike to occur, there must be a collective group of employees or union in place. A single employee walking out wouldn’t count as a strike.
If there is a union in place, the union can’t decide to strike on its own, nor can union employees strike without union approval. Instead, the union leaders have to call for strike action and put it to a vote. If the majority of the union (typically 80%) votes in favor of striking, then a strike can occur.
Strikes are a protected collective activity in the NLRA as long as the strikes are lawful. This protection means employers can’t retaliate against or discipline employees who participated in the strike.