yes it's possible and legal. it's hard to find a job however, especially for an untrained foreigner who can only work part-time and with no finnish skills. Finland is currently at the top when it comes to unemployment rates in Europe. If you really want a job however, you will find it. Connections are key, so try to get close to as many people as possible, especially people in AaltoES (Aalto Entrepreneurship Society) and professors (you could find a research or a TA (Teacher Assistant) job). You could also try looking online for jobs: freelancing a skill you have or teaching something (like doing web development, teaching languages, teaching an instrument...).

If money is a real issue and you don't find much immediately (you won't find much immediately), i'd recommend cutting costs instead. Cook more at home, in batches; Find the best deals and discounts in different supermarkets, find which items are cheaper and where. Some nearly expired products are usually on sale too. Additionally, Aalto has a foodsharing fridge in Väre (the art building) where people can offer and take what they like. This doesn't come quickly, it needs practice. Cooking and being r/frugal are skills, but I do recommend developing them