What the diagram shows. The nucleus sits in the centre, and each ring is one shell (also called an energy level). The shells are drawn as circles at fixed distances, but this is only a way of keeping count: the electrons are not really circling the nucleus on tracks, and the rings say nothing about where an electron actually is. What the diagram does record correctly is how many electrons there are in each shell, and that the shells further from the nucleus are higher in energy.
How the shells fill. Electrons go into the lowest-energy shell that has room. The first shell holds up to 2 electrons and the second up to 8. The third shell takes 8 electrons before the fourth shell starts, so potassium is 2,8,8,1 and calcium is 2,8,8,2. The electronic structure is written as the number of electrons in each shell, innermost first, separated by commas.
Why it stops at calcium. From scandium (element 21) onwards the "8 then start the next shell" rule breaks down: the third shell can actually hold 18 electrons, and after calcium the extra electrons go back into it while the fourth shell already has 2. Explaining why needs the idea that each shell is split into sub-shells of different energy, which is A-level work. Every element up to calcium follows the simple rule exactly, so this is the natural place to stop. The A-level Electron Configuration simulation carries on to krypton.
Ions. Removing electrons takes them from the outer shell first, since those are the least strongly held; adding electrons puts them into the lowest shell with space. The simulation lets you remove or add as many as you like so that you can see what does and does not happen in practice: sodium loses one electron to reach a full outer shell and stops there, because taking a second electron out of a full shell is far harder.
Drawing convention. Electrons are placed singly at the top, bottom, left and right of a shell and then pair up, which is how most GCSE textbooks and mark schemes draw them. Exam boards accept dots or crosses; the toggle switches between the two. Both are the same electron, drawn differently.