How to navigate a 1d array in a 2d manner?

Question

I've got this sample code below. I understand cells is being made in a 1 dimensional manner, since the compiler stores 2d arrays in memory row-by-row in 1d. What I don't understand/can't visualize is how to access elements or change positions.

1. I know board->cells[i + (20 * (10 - 1))] = FLAG_WALL; is used to set flags for the bottom most row. From the format it looks to be using a formula but I am unsure of what it is. The formula also seems to change for the right and left edges. (board->cells[i * 20 + 20 - 1] = FLAG_WALL;)

2. Looking at the line board->cells[20 * 2 + 2] = FLAG_SNAKE; it seems cell[42] is where the snake flag is spawned. What would the arithmetic look like to shift its position by 1 cell up, down, right, left? When imagining cell[42] as a part of a 1d array in memory I can't figure out the arithmetic nor the resulting cell number after a shift such as 'up'.

board_init_status_t initialize_default_board(board_t* board) {
    board->width = 20;
    board->height = 10;
    board->cells = calloc(20 * 10, sizeof(int));

    // Set edge cells!
    // Top and bottom edges:
    for (int i = 0; i < 20; ++i) {
        board->cells[i] = FLAG_WALL;
        board->cells[i + (20 * (10 - 1))] = FLAG_WALL;
    }
    // Left and right edges:
    for (int i = 0; i < 10; ++i) {
        board->cells[i * 20] = FLAG_WALL;
        board->cells[i * 20 + 20 - 1] = FLAG_WALL;
    }

    // Add snake
    board->cells[20 * 2 + 2] = FLAG_SNAKE;

    return INIT_SUCCESS;
}