How to make the most of an online research internship?
Keep a weekly (or even more frequent) meeting with the professor or supervisor to discuss progress, which will help keep you accountable. Furthermore, participate in virtual seminars, talks, and happy-hours that may be made available to you.
Most importantly, be persistent in your work. The high-school and undergraduate students that I work with who make the most of research opportunities do so by working hard and taking the opportunity seriously. The ones who do not tend to not put much effort into the experience. Additionally, unless the contrary is explicitly agreed upon, you will need to take initiative in your research. Do not just complete the tasks you are given; think about how your work fits in the broader research context, read papers and ask your supervisor questions, think about what additional experiments you can do, etc.