Learn Spanish (Latin America): Bienvenido, señor García.

Yes. The word bienvenido acts like an adjective, so its ending changes to match the person or people you are welcoming. Because Mr. García is one man, it is bienvenido. If you were welcoming Mrs. García, you would say Bienvenida, señora García.

Why isn't señor capitalized like "Mr." is in English?

In Spanish, titles like señor, señora, and doctor are written with lowercase letters when spelled out completely. However, if you use the abbreviation (such as Sr. or Sra.), the first letter is capitalized.

I learned before that we have to say el señor. Why is there no el here?

You only use the definite article (el or la) when you are talking about someone, for example: El señor García está aquí (Mr. García is here). When you are talking directly to the person, as in this greeting, you drop the el.

How does this greeting connect to using tú or usted?