You can use beer from any brewery on the board that's connected to your industry via canals/rails, including your own breweries. You don't have to buy beer from the market if you have access to brewery beer through your network.

If you cannot obtain the required resources (coal, iron, or beer) from either the market or connected industries on the board, you cannot perform that action. You must choose a different action or pass if no valid actions are available.

When you develop an industry, your income increases immediately and you move your income marker right away. However, you only receive money during the income phases at the end of each round, not immediately when developing.

Yes, during the Railroad Era you can build railroad links even if canal links already exist between the same locations. Both types of links can coexist and function independently for network connectivity and scoring.

Each Sell action allows you to sell from only one industry. If you want to sell from multiple industries, you need to take multiple Sell actions (either in the same turn if you have multiple actions, or across different turns).

No, you only score points for your own industries and links during network scoring. Other players' industries in your network don't give you points, though you can use their resources (like beer from their breweries) during the game.

When you discard cards (either from playing them or from hand limit), you can choose the order they go into your discard pile. This gives you some control over which cards you'll draw back when your discard pile gets shuffled into your hand.