Learn English – difference between “on campus” and “at campus”

Simply put, "I DO know this" is used to emphasize that you know the thing you're talking about, whether or not you know anything else.

Look at the OP's entire sentence:

I am an almost newbie at electronics but i do know a few things

Here OP says that even though he is new to electronics, but he still knows a few things. Therefore, "I DO know" is used to emphasize the point that he "does know" a few things, in spite of him being an amateur.

On the other hand, "I know" is just a simple expression stating you know something. It doesn't talk about something which you might not know.

Suggest to take a look at this question. The answer explains the phrase "as far as I know" (among other things), and uses this sentence:

Similarly, as far as I know means to the extent that my knowledge reaches—that is, “I do not claim to know everything about this topic, but what I do know is this ...”

See here how "I do know" is used to imply that you do not know everything about the topic, but you something, and that something is...

What you learned there is absolutely correct. Most of the time, we do use the preposition in after the verb invest. The preposition into is also possible, but in is used a lot more commonly. However, the to that you're talking about there is not a preposition at all. It's something called an infinitive marker which goes along with the verb introduce and indicates that the verb introduce is being used in its infinitive form (for example, to buy, to see and to watch are all the infinitive forms of the verbs buy, see and watch respectively). One of the goals of using verbs in their infinitive forms is to specify the purpose of an action. For instance, to see in I went there to see the man specifies the purpose of the action of going there. I went there for the purpose of seeing the man. Consider this sentence:

They invested lots of money in the company to make even more money.

Now, I will ask you the following question what was the purpose of them investing lots of money in the company? The answer you should give would be the purpose of them investing lots of money in the company was to make even more money. Do you see how this works?

So, we could rewrite your sentence like this:

However, last year, my university invested a significant amount of money (in its infrastructure) for the purpose of introducing a greater number of healthy food options at its cafeterias.