What is this weird (Flex layout) C++ code?

Obviously, I am way behind in modern C++ syntax past my initial courses. While looking at the WebKit source code in order to understand the flex layout, I see this code:

  auto performMainAxisLayout = [&] {
            computeLogicalWidthForFlexItems(flexItems, lineRange, availableLogicalHorizontalSpace, flexRects);
            distributeMarginAutoInMainAxis(flexItems, lineRange, availableLogicalHorizontalSpace, flexRects);
            justifyFlexItems(flexItems, lineRange, availableLogicalHorizontalSpace, flexRects);
        };
        performMainAxisLayout();

        auto performCrossAxisLayout = [&] {
            auto availableLogicalVerticalSpace = lineHeightList[index];
            computeLogicalHeightForFlexItems(flexItems, lineRange, availableLogicalVerticalSpace, flexRects);
            distributeMarginAutoInCrossAxis(flexItems, lineRange, availableLogicalVerticalSpace, flexRects);
            alignFlexItems(flexItems, lineRange, { lineTop, availableLogicalVerticalSpace }, flexRects);
            lineTop += availableLogicalVerticalSpace;
        };
        performCrossAxisLayout();

This can be seen here[0] for more context. My question is - what is this [&] thing? And I cannot find the method named performMainAxisLayout() or performCrossAxisLayout() defined anywhere in the Layout class either. My assumption was that here, a structure/literal class is being built and the methods are on that class but the Layout class don't have them either.

Where can I learn more about this syntax or modern C++ quick overview in general?

[0]. https://github.com/WebKit/WebKit/blob/main/Source/WebCore/layout/formattingContexts/flex/FlexLayout.cpp#L506-L536

EDIT: Fixed title typo.