I will be starting at the job the next summer (graduating next summer) and I too thought the same. If I didn't use the company's infrastructure or their internal technology or do the work while on company time (9 to 5) - then they should not be able to claim my work.
TL;DR If you could not have built the product without being an employee of the company, it probably belongs to them.
eg. Maybe you use Microsoft's latest internal only HoloLens prototype to build a product. You couldn't have done that if you weren't a MS employee - so they can claim it.
But if you used the dev kit and publicly available tech and features then they shouldn't be able to claim it.
TL;DR If you could not have built the product without being an employee of the company, it probably belongs to them.
eg. Maybe you use Microsoft's latest internal only HoloLens prototype to build a product. You couldn't have done that if you weren't a MS employee - so they can claim it. But if you used the dev kit and publicly available tech and features then they shouldn't be able to claim it.