I worked in Switzerland, the 13th month wasn't a bonus, they simply took your salary, divided that by 13, that was your monthly payment and you got 2 of them in December.

Also many bills are due on the first of January for the whole next year e.g. insurance. So it was kinda like a holiday and large bill windfall, but it wasn't additional to salary.

That was just one company, maybe other countries or employers are different.