You may not be in a regular band/ensemble but available for one-off recording or gigging/performance opportunities, in other words as a session musician. To succeed as a session musician or backing singer you must excel in three areas:

• musicianship
• networking and people skills
• business nous

Primarily you must be an excellent musician so focus on honing your craft through vocal training, instrument proficiency, and studio experience. Perform live frequently to build confidence and versatility, and maintain a strong online presence with professional demos and videos. Network actively by joining music communities, attending industry events, and building relationships with fixers, producers and other musicians. Always be professional, punctual, and open to feedback, while taking care of your voice and health. Stay informed about industry trends, understand music contracts, and manage your finances well to ensure long-term success.

The Musicians Union has useful advice here https://musiciansunion.org.uk/working-performing/recording-and-broadcasting/working-as-a-recording-session-musician