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ASP.NET Core
ASP.NET Core is a cross-platform .NET framework for building modern cloud-based web applications on Windows, Mac, or Linux.
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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
I would recommend following this roadmap https://github.com/saifaustcse/dotnet-developer-roadmap I think it summarises everything a great developer in general must know about.
I think you should learn how to make a web api with asp.net connected to a database, and maybe (and if interests you) a front-end framework that interacts with that api. That seems like the de-facto standard in .net development nowadays
NOTE:
The number of mentions on this list indicates mentions on common posts plus user suggested alternatives.
Hence, a higher number means a more popular project.