I'm writing a book (
https://opinionatedlaunch.com/) on building and running subscription-based mobile apps (so, SaaS) from a technical implementation perspective (as opposed to a marketing or "startup" book). The book's case study is an app that generates PDF certificates for webinars. I chose that idea because it's straightforward, so people can focus on the book's lessons (e.g., "You want Google login like in the app? Check page 15").
Generating PDFs is very easy. You can either call some other SaaS' API (free plan available) or roll your own using Puppeteer (this is what I use). BUT to make it a full-fledged SaaS, you need to do more work than just that I-can-build-over-the-weekend thing:
* Signups and logins (email/password login, forgot password, Google login, Apple login if you use Google login on iPhone).
* Onboarding screens for new users.
* Show stats to make people think it's worth paying for your app.
* Subscription: handling trials, integration with App/Play Store, and downgrades/upgrades.
* Automatic sub-domain and custom domain tenancy (e.g. acme-inc.myapp.com).
* Email: SPF, DKIM, DMARC, email templating, copy, compatibility with different email clients.
* Push notification.
* Android, iOS and Web versions of your app.
* Integration with Zapier so your app won't be a silo.
* Even things you probably don't think important: Store page with nice screenshots and good copywriting, a nice app icon that doesn't look like it's designed by a programmer.
* Oh God, the website for your app, the dreaded marketing copy, a pricing page, knowledge base, and blog.
* Chat bubble in the bottom right of your website and email routing for customer support.
* Social media presence *shudder*. If you think it's as simple as creating account in X and setting reminders to post content, think again.
To answer your question: "Isn't all SaaS just wrappers?" I don't know if it's "just" wrappers, but it's a VERY thick wrapper :D