These questions regarding the use of Zapier occur frequently.
Can we create an integration user (instead of a named admin user) in reteach for configuration in Zapier?
Yes, that's easily possible. Each tenant should have a unique email address.
How does it work with different tenants?
Again, each tenant should have a unique email address.
Is it possible for reteach to know in which tenant, for example, new users should be created?
The logic for that should be built in Zapier. Since various reteach accounts/tenants can be set up in Zapier, you can easily create this branching with Conditional Logic. See, for example, here:
https://zapier.com/blog/zapier-paths-conditional-workflows/
https://community.zapier.com/featured-articles-65/3-ways-to-add-conditions-to-zaps-22333
How can we set it up so that newly created users automatically receive a certificate for an assigned course?
By ensuring that the certificate setting "Enable by default" is set for each course.