As a rule, dividends are not automatically reinvested. Distributions are always credited to your balance, so that this amount is available for future transactions.
A special case is represented by ETFs, which often end with the letter combination (Acc). This stands for accumulating (German: thesaurierend) and means that the respective ETF issuer directly reinvests the distribution back into the underlying ETF. Please always check the "Key Investor Information" to ensure this also applies to your selected ETF. Dividend distributions resulting from holding a stock are generally always credited to your broker balance.
You also have the option to set for yourself that dividend distributions are always and automatically reinvested for all securities.
If you wish to do this, select the respective product and go to the product details (via the gear icon). Then click on "Distributions" and select "Auto-Reinvest". This feature is free of charge.
If necessary, you can also reverse this setting the same way and change it back to "Payout".