There are two straightforward ways:
1. PWA, this way your user would be able to install the app from browser
2. Webview which is basically a web browser inside an app.
Both ways suck, if you have a big budget ($100.000) and want to build an app that behaves like an app and has all the features of an app you should hire a company to build your app from scratch.
I would waste time on building webview app.
There is also React Native technology for building mobile app, which works fine, maybe you could re-use some of your website code to build the app in this technology. It might be a good trade-off if you already have the website built.
I think that focusing on the website and then when you have enough profit to spend on a good app, build a real app. Good luck!