in addition to; as well as; besides

"In addition to" can add a sentence after it;
besides is a preposition and adv. It can be used as a preposition, followed by noun.

As well as is a phrase, followed by noun.

1. In addition to what I just described above for you, you can look at other materials.

2. Besides, I'd like you to know that I will be late for next meeting.

3. She is very good at the spelling as well as Math.

Bob

in addition to表示除了什么东西还有什么东西，相当于besides....表示补充说明。
as well as,也.