"Welcome to" is used when:
* Introducing a place or group: "Welcome to the party!" "Welcome to our team!"
* Describing the purpose or function: "Welcome to the world of online learning."
"Welcome in" is used when:
* Inviting someone inside: "Welcome in, the fire is lit."
"Welcome" alone is also correct in many situations:
* Greeting someone in general: "Welcome!"
* Expressing happiness someone is present: "Welcome, we're so glad you could make it."
So, it's not about which is grammatically "correct," but which phrase is the most appropriate for the situation.