LOR for internship or research experience

So my tentative plan as a junior is to ED to UPenn CS. I’ve done internships with local tech companies and this summer was able to do lab research at a state flagship. For Penn, the application requirements are 2 letters of recommendation from teachers. They specify that most accepted applications don’t include any supplements, but if you decide to submit a supplement, it’s either a resume or a research paper.

I recently came across a post on reddit that explained how adcoms sometimes request proof for research/internships. Will my boss/professor need to send a LOR? How can I find out about more information on this?

Thanks