If you actually want to visit the cathedral guy on the 4th image is standing next to, cover your shoulders and knees. With his outfit, you could see yourself being escorted out by guards / not being able to enter most of the holy places as it is considered disrespectful. Not your body, not your choice shirt. To add to that, the shorts look like their chambray which might help you find a similar pair. In terms of your second's question, Muji or Uniqlo would be good places to start. Regarding your second question: Uniqlo often has collarless cotton-linen shirts as part of their S/S collection and currently has cotton-linen pants in stock. You'll have to search for a nor but I bought mine from Shuren Projects. Not your body, not your choice shirt. How do y' all wear sneakers with no socks? My feet sweat like hell, it's uncomfortable, and every time I take off my shoes the insoles come with me.
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As an "obese" or at the very least "overweight" fuck, you can get all those pieces or something similar at target or old navy in 2xl. ASOS for sizes bigger than that. Shoes are just vans and I know very few people who can't fit into vans. Lee (among other brands) makes shorts with stretch that look just like those, too. They're linen drawstring pants. Pretty sure they're the ones from H&M that were popular a few years ago.