Short answer — the labels most commonly associated with celebrity engagement looks are a mix of bridal/couture houses, big luxury fashion houses and a few contemporary ready‑to‑wear names. Those most frequently seen in engagement announcements, proposal photos and engagement parties include:

• Bridal / couture specialists: Vera Wang, Monique Lhuillier, Oscar de la Renta, Carolina Herrera, Marchesa, Reem Acra, Elie Saab — known for romantic, bridal-ready silhouettes and custom gowns.
• Major luxury houses: Valentino, Dior, Chanel, Gucci, Givenchy, Alexander McQueen, Saint Laurent, Dolce & Gabbana, Celine — favored for red‑carpet gravitas and couture or bespoke options.
• Contemporary/modern occasionwear: Self‑Portrait, Zimmermann, Reformation, Rotate, Mônot, Aje — popular for Instagram-friendly, fashion‑forward engagement outfits that read bridal without being full wedding dresses.
• Tailoring/minimal: The Row, Stella McCartney, Max Mara — chosen for elegant suiting or pared-back luxury looks.
• Accessible/high‑street picks celebrities sometimes wear for low‑key or budget‑conscious announcements: Zara, H&M, ASOS, & Other Stories.

Why these brands? Celebrities and stylists pick them for their signature silhouettes (romance, lace, corsetry, dramatic sleeves), couture/bespoke services, red‑carpet credibility, or because they photograph well on social media. Increasingly many couples also commission one‑off looks from smaller independent designers for exclusivity.

If you want, I can give concrete celebrity examples (who wore what) or recommend brands by budget and style (romantic, modern minimalist, sequin/party).