There’s a couple ways to do it, depending on how much you do/don’t want it to look like WordPress. In both scenarios you have to mentally translate what you want to do into WP architecture (e.g. you want a new model? In WP that’s a custom post type, etc.).

The “easier” way is to create custom post types for each model you want, and use Advanced Custom Fields (plugin) for the data fields. Users log into the WP backend and can CRUD the custom post types. With ACF, pretty amazing things are possible (inter-model relationships, uploads, etc, and all done with amazing UIs for each type of field).

The “harder” way (if you are trying to make your app appear less like WP) is to get a plug-in that hijacks the user login process and admin, then customize the front-end to display the CRUD fields/controls (again using custom post types and ACF) when users are logged in, so your users never see the WP admin.

Both ways (and more, now that WP also has a new-ish full-featured rest API) are possible, however the standard WP caveats apply: the more plugin-spaghettini you introduce, the more unexpected issues you will face. I’ve been meaning to write up a guide, need to do that one of these days...