Carrer advice
I am currently intern at startup which is based on billing software works with .net the work culture is pretty toxic over here and stressful any advice on which tech stack to learn to upgrade skills and move from here
Billing software is a good domain with vast applications, gain the knowledge around it and if you show-case the domain knowledge you have in it, surely you will bag a better opportunity irrespective of the tech stack
You are already on .net it's better to learn .net core and upgrade yourself with this upgrade you will be eligible to work on all platforms. So learn .net core and move out.