It takes repetition and practice to understand all of this.
Here are a few statements to keep in mind.
Be sure not to confuse current and voltage. Voltage is the difference in charge that puts pressure on electrons to move. If there is a complete circuit, then electrons will move. We call this electron movement “current.”
The current is the same at any point in a series circuit, before or after loads.
If there are multiple loads in a circuit, and current is flowing through it, there will be a difference in voltage before and after each load. This is called “voltage drop” and we’ll cover it in more detail in Unit 8. The sum of the voltage drop across each load will equal the source voltage.
Parallel circuit are really just series circuits that are tied together to the same power supply in some way. Each individual branch behaves like a series circuit. The videos at the end of unit 4 are good at discussing this.