You need to choose a domain. Find software and libraries in that domain and then work off of that. "Modern C++" can mean alot of things. Most people conflate this with the STD library, but alot of people don't use that. Core language feature wise it's not that much. Some "Modern" c++ domains don't even use the STD.
This is similar to asking "What is the best way to learn modern Java", where the answer will vary vastly based off what whether you're doing embedded, desktop, server, serverless, websites etc.
Possibilities for "modern" C++ domains. HPC, Machine Learning, Finance, Gamedev, Crypto, Scientific... Also keep in mind most are going to be writing C with Classes and target C++11 at most.
