I am trying to get information about a customer using the REST API of Shopware 6 (using this API). Therefor I need a ContextToken for that user. The only way to get a context token for a user is to log him in, but for that I need the password of a user which I do not have. Is there any way to get the ContextToken of a user with only his email address or hist customer id?
Shopware comes with 2 different APIs, the Store-API and the Admin-API. Refer to the docs.
The Store-API is there to build a storefront and thus you need to authenticate as the user using that API.
When you build custom integrations you should use the Admin-API, as that API can be used for CRUD-like operations on all entities of the system. For the admin API you can generate an API-Token and use it to query data of all customers.