I’m not familiar with American geography, so the little thin threadlike patches of lands dividing the three lakes, are those accessible? Can people go across the lakes via those barely visible land bridges?

these?

those aren't land bridges, they're bridge bridges!

the southern one is the Mackinac Bridge, which is the longest suspension bridge in the western hemisphere.

it's fucking ENORMOUS. it's about 5 miles long and the deck is 200 feet over the water. the bridge towers are 550 feet tall.

it honestly breaks your brain a bit to look at it, but if you want to cross from Lower to Upper Michigan or vice versa, it's your only choice.

and the northern circle is the Sault Ste. Marie International Bridge, ...