The Quantum Realm has resources and civilizations that support life.
Ant-Man and the Wasp: Quantumania (2023) provides insight into Janet Van Dyne's survival in the Quantum Realm. The film reveals that the realm is far from empty or inhospitable.
In Quantumania, when Scott, Cassie, Hope, Janet, and Hank are transported to the Quantum Realm, they discover a world with native inhabitants. It is home to various intelligent beings and even civilizations. These inhabitants have access to essential resources like food and water, which explains how Janet survived her 30 years there.
The film also shows that Janet lived among the natives of the Quantum Realm, adopting their ways of life. It's reasonable to assume that her mysterious attire and equipment, such as theher hood, spear, and makeup, were likely acquired from the local inhabitants.