She doesn't, she's a subcontractor
Dr. Stelline emphasises that she doesn't actually work for Wallace, rather that she's a subcontractor of his.
In her discussion with K:
K: You work for Wallace?
-Paraphrased from memory
She runs her own business developing memory implants, which she supplies to Wallace.
You may have noticed that when K asked her about it, she was quick to reply with "subcontractor", as though she herself wasn't comfortable with the idea of working for Wallace herself.
As for how she got that facility, it's unclear, however, it's safe to assume that either:
- She worked as a successful memory manufacturer already
- So she paid for it herself, slowly building up her wealth like any other person
- Her 'parents' were well off
- And left her with enough wealth to either get started, or fund the special room of hers
- The Resistance ('Replicants Underground') helped her
It's all quite fishy to say the least, and to me, it seems like there's more to her story than just what she tells
K. See How could Ana Stelline survive childhood?