Icons:

How and when to use them/ icon sets. Usually convention works best. What makes the most sense works. Look at olympics icons. Icons transcend language. Icons have a role in user interface. For navigation always use labels for accessibility. For smaller screens icons will need to be simplified.

Styles of icons:

Scale: sized. Different screens

Keep in mind that when scaling down the quality can get lost. Design a range of icons in different scales. Design system. Readable at all sizes.

Harmony:

keep everything cohesive. Colour, style, line thickness, style. Differentiate, the same but different. Break everything down into basic shapes.

Creating words out of icons: Word marks.

Icons/ detailed and simplified:

• Different types of icons for different uses.
• UI icons are usually very simplistic.
• Functional icons
• Badge/picture like icons.