Your bitcoin is always delivered to the default address saved in My addresses. You can change it at any time:
On the Buy bitcoin page, tap the pen icon on the destination card to open the address picker and choose another saved address as your default.
To add a new address or edit existing ones, tap Manage in the picker, or open the account menu and select My addresses.
There you can add on-chain or Lightning addresses with Add address, edit them with a label you will recognize, and set which one is the default.
For security, you will receive an email notification whenever your payout address is changed.