For SEO, multi-page websites are almost always superior. Each page is an opportunity to rank for a different set of keywords. A one-page site has, in practice, a single URL competing for every term, which drastically limits its organic potential.
One-page sites can work for projects with a single, specific goal, such as campaign landing pages or artist portfolios. But for companies that want to generate organic traffic and show up in AI engines, multiple pages with specialized content are essential.
The recommendation is to have dedicated pages for: each service offered, each segment served, each important frequently asked question, a blog with regular articles, and institutional pages (about, contact, team). This structure maximizes ranking opportunities and provides more "contact surface" for AI engines to cite.