How can I build an internal firm portal and client-facing app (with onboarding hub, forms, and niche landing pages) without a developer?

One member demoed an app they built themselves that serves as an internal staff portal plus a client onboarding hub, with forms and even custom landing pages for events, all built directly inside the same tool. They also built their firm's websites this way, including a niche site for their therapist-CPA client base. The member described it as 'pretty slick' and noted it let them consolidate tools that previously would have required separate website builders or form software.