Largely propriety tech-stack Eng: like FANGs and banks. Largely open-source tech-stack Eng: like most non-FANGs. Examples: Build Systems, CICD, SCM, Configuration Management, internal frameworks, etc.
Mid sized company with some tooling but mainly open source systems. Someone who open sources their systems as well.
Are you just starting off your career? I think it may depend where you're at in your career. If you're starting off your career I don't think a company with a lot of proprietary systems is ideal.
Care to elaborate?
If you only use proprietary in-house built tools when you go to the next job it's hard to talk about that tool to someone that knows nothing about it. The interviewer would not even be able to go back to their desk and Google what you were talking about. I use proprietary tools here in Amazon and I'm fairly confident that some non-faang company could care less about our internal Amazon tools. If you use tools that most the industry is using at least there's some commonality. If you're already experienced I don't think it's as much of a big deal but if you have no experience and you come out of college and all you use is proprietary tools I think some employers would be turned off by that. But I could be making a bigger deal out of this than it needs to be.