That means the head of your dick must be visible.

A banya hat

An ice cream cone? To comedically place it between your privates and the camera?

That's the international symbol for "Circumcised dicks only"

Jaa mitähän ihmettä? I've been to many public saunas in Finland and almost all of them, if not all, prohibit the use of swimsuit/-pants. At least the ones that are in public pools/swimming halls. And the reasoning being as you said, hygiene.

Edit: whoops my bad, you were talking about mixed gender Saunas. You might be correct after all

Can confirm re: Germany. It's often explicitly framed as a hygiene issue. That said, there are saunas where you may wear stuff, it's usually designated. Plus you have "women only days" in a good number of public saunas.

In Russia it's also common to eat dried fish and drink beer/sometimes vodka in the room next to the sauna.

A swimsuit is quite common in Norway. Basically you ask yourself the question "am I going to make anyone uncomfortable". If it's single gender and people are not extremely shy, you generally go with only a towel, but nobody is really going to care. If you're a gender mixed group of friends that don't know each other that well, you might prefer putting on a swimsuit in order to make sure people feel comfortable and included.

From my experience the Swedes are the same.

This is based on private saunas with friends. In public mixed gender saunas I don't think I've seen anyone go naked, but I'm sure certain Finnish tourists would and nobody would mind.

Can confirm the german part: In my gym there recently was an outrage because a Muslim member went to the Sauna with bathing clothes multiple times, which in the end resulted in his contract being terminated. Being naked in the sauna is almost the law here (but in the end nobody cares if you wear your towel or put it below you. Just remember: No sweat on wood!)

And Great Britain should be orange. Everywhere here would be 100% fine with you wearing a bikini or swimming shorts in a sauna as long as you aren't completely nude.

Not only wrong - I can't look at the image without turning off my vpn

I consider it as a glorified Imgur. If you want to upload the images to catbox.moe or anything else and share the link, feel free, I would update the post!

How do you reproduce?

Agreed. I don't need the sweat infused into the wood and me sitting on top.
There may be rules to shower beforehand but some will forget to choose to ignore it.

Sauna hat.

In Finland people are told to not wear swimsuits to sauna not because you absolutely have to be naked but because if you've been in a chlorinated pool the chemicals vaporize in the sauna and are harmful to breathe.