Can someone from another device see what I'm doing on my phone?

Could you please elaborate on the potential risks associated with someone from another device potentially being able to view what you're doing on your phone? Are there specific vulnerabilities in mobile devices that could allow unauthorized access? How can users protect themselves and their personal information from such intrusions? Is there a way to detect if someone is remotely accessing your device, and what steps should be taken if you suspect your phone has been compromised?