@Emiel1 Well, I'd prefer to be perceived as rude rather than expose myself to a deadly virus just because a clueless guest wanted to show me something on their phone.
But it doesn't have to be rude, just say something like "No offense, but I take the social distancing precautions re COVID very seriously. Maybe you could email me what you want to show me on your phone?"
Unfortunately, young people being lackadaisical about the virus is a big issue in many places. They seem to think they're invulnerable, even though there have been many young people, otherwise healthy, who have died from COVID or gotten infected and ended up with chronic health issues. Young people are the major vectors for infection now. It's like they have to infect their grandma and have her die before they "get it".
I know you need to earn money, but by renting out shared homes to people who aren't taking the recommended precautions, you are contributing to the virus spread, even though you yourself are being responsible.
Have you considered renting out the places as entire homes, to one set of guests, rather than unrelated people?