In QGroundControl there is the option of setting the exact position of the base station. How can I get the final Position and then use it for the next day?

You can find the answer in the PX4 User Guide.Here are excerpts from the article

You can save and reuse a base position in order to save time: perform Survey-In once, select Use Specified Base Position and press Save Current Base Position to copy in the values for the last survey. The values will then persist across QGC reboots until they are changed.