Upon Information and Belief | Personal First-hand Knowledge
There are two very different levels of assertion that someone can make.
The lower level is when the one making an assertion has reason to believe, and does believe, that the assertion is true and accurate. The affiant is testifying about their sincerely-held beliefs, but obviously, people might sincerely believe something false. We call this relatively-flimsy type of assertion "upon information and belief".
The higher level is when the one making an assertion has personal knowledge, first-hand knowledge, having personally witnessed something. The affiant is testifying as to what they saw with their own eyes, heard with their own ears, felt with their own senses, or otherwise personally perceived. We call this "personal knowledge" or "first-hand knowledge".
When testifying as to what one heard with one's ears, there is a fine line of "hearsay" involved, so the affiant should pay attention. For example, in recounting a bank robbery event, someone who was standing in line and heard yelling from behind them could not testify that they heard the defendant yell "everybody get down!". That would be hearsay; however, the witness could testify to hearing a loud voice yelling "everybody get down!" and that would be perfectly admissible. The witness could even testify that they turned around to see who had yelled, and it appeared to have been the defendant... or that the witness believes it was the defendant.
In most jurisdictions, a criminal complaint, while technically in the category of a "charging instrument" or "accusatory document", is constitutionally inadequate for commencing a criminal cause of action. Most constitutions — state and federal — require that criminal court cases are only commenced by the presentment of "primary pleadings" (an indictment or an information), along with the sworn complaint which was the basis of the primary pleadings.
But even in the rare situations where a criminal complaint alone might be deemed acceptable for commencing a criminal case, still, a criminal complaint that is sworn out upon the affiant's belief would only be sufficient to get the suspect/accused before a magistrate, for a probable cause determination hearing. At the probable cause determination hearing, the rules of evidence would kick in, and that relatively-flimsy type of assertion would be inadmissible as evidence, so the affiant who made that complaint would then need to be examined, under oath, by the magistrate, to elicit some actual admissible evidence: the underlying facts and reasons supporting the affiant's belief.