Steem can be an investment (if you powerup or save or sell if the prize is high and buy more if the price drops).
SBI the same

UVF and SBI upvote on average all types of posts but a certain "power" should be reached before you receive an upvote (see it as your VP (see https://www.steemworld.org/@henryclive). The more shares you have the higher and faster your upvote.
You will not receive an upvote for everything you post on one day which most Steemians will also not do or are not able to (that's why one post per day at most is advised).

• If I remember well the minimum is 12.

• The more shares the higher your upvote. If you post again (same day or next day) the vote will arrive but be lower because you have to receive that "VP". This is the same for SBI as it is for UVF.