I interned at Bloomberg last summer.

Tech stack is relatively old, yes. But many teams are really pushing to change that (not all are, though). What you will get to work on is almost entirely dependent on the team you will join, which they will mostly pre-decided for you. Mostly the only control you will have is which section of R&D you want to join (Infrastructure, Foundational, etc.) A lot of the development is in C++, but there are a lot of teams also doing work in Java, Python, Javascript and other languages. There is one point which I would like to stress: old/outdated/internal doesn't always translate to a bad tech stack or a bad dev experience; there are plenty of good engineering challenges to solve, regardless of the stack (more on this later). The infrastructure team is slowly but steadily updating stuff, and teams handling their own infrastructure generally have a bit more leeway to use new tech. As an intern, you have little say on what exactly you'd like to work on; so if you are looking at a very specific work role, Bloomberg might not be for you.

Bloomberg is a lot more than just plain fintech. It has hard-core engineering problems to solve: real-time data (market ticks from many many different places), scalability (news, Vault, financial data, all these need lots of space and they have extremely hard performance guarantees to hold up to), security (obvious), user and developer experience(their own APIs, integrating with other data providers' API, creating new functions for users paying upwards of $2000 a month for one user), internal tooling, machine learning, search... I could go on. Up to you to decide whether working on all this is relevant to other companies or not.

A lot of interns worked with the latest technologies/stacks/problems, which are definitely relevant today, and I know for sure they had a great time. I, however, was put in a team which admittedly wasn't "cool" (can't reveal which, too identifying). All the devs in it had more than ten years of experience, and most of the programming from outside looked like python/perl scripting. It took me more than two months to realize the massive scale of the design problem they were working on and what exactly were the challenges they were facing (their decisions would directly affect R&D's development workflows). Coming in as a narrow-minded student knowing little of software design/architecture, it made me realize software development is much more than the latest platform fad or the newest open-source storage solution. My personal project was certainly difficult; my stack and my task wasn't anything I had worked on before. The value for me in that internship was more of thinking deeply on software design given a limited flexibility I had, getting to ask lots of questions to the team (they were awesome developers) and figuring out how things actually run within large company tech (they had a great view of most of the infrastructure given the work they were doing). I ended up enjoying my internship, have an offer, and I'm getting calls from most major tech companies for interviews.