There is a 'the' missing in English 'fridge magnets' ... Should be 'where is THE salt'
What sound does "ь" make exactly? All I hear is something like "seel" with nothing after the "л. "
It modifies preceding letter (softens it). Compare Spanish N and Ñ for easy reference. In Ukrainian almost any letter can be softened in a similar manner.
What would сіл sound like without ь? If that question is answerable over text
My Ukrainian partner tells me that without the ь your tongue ends with its tip on the back of your front teeth. With the softener the middle of your tongue ends touching the gums behind your teeth, with your mouth slightly more open. As others have pointed out it's like n vs ñ in Spanish
"Сіль" should always be transliterated as "Sil" ( add < ' > or whatever else they may use for "ь")