> The article doesn't mention any details in that regard.

The whole point here is that you don't. You have dozens or hundreds of separate apps. They can use whatever framework they want. If you want to migrate framework, you just rewrite them from scratch, without any impact to another. It solves your current problem by making it a non-issue.

Consistency comes from the design system and style guide. If you have 2 frameworks, you have to start by having multiple implementation of the style guide. As long as the implementations are accurate, the look and feel of the apps will be the same even if they use different implementations. It has to be done well though.

As for Angular 1.x/React, the solution Ive used in the past in environment that didn't have micro apps was to build tools that let me React.render arbitrary react components inside of any Angular directive, and build a React component that let me render arbitrary angular directive anywhere, so I could migrate part of apps starting at any point without affecting the rest. I unfortunately don't have the code anymore, but it wasn't very hard to do.

You could fetch and cache data into local storage to prevent re-fetching. Depending on how you are set up you could have both apps loaded at once and display one or the other (perhaps lazily loafing the one you don’t need upfront)

We are doing this for simple data where we can but unfortunately having state in multiple locations (backend, memory, localStorage) means lots of cache complexity. How do you refresh and invalidate when the data are in so many places? We've opted to keep data in only 1 place as much as possible due to this exponential complexity.

We also have to be careful what we store in localStorage since it is relatively insecure.

Check out single-spa

https://github.com/CanopyTax/single-spa