Insiders merely need to file a Form 4 https://www.investopedia.com/terms/f/form4.asp
with the SEC within 2 business days after their transaction (buy or sell), though I have periodically seen some go well beyond the 2 day filing rule and there is never any enforcement action. There is no stipulation regarding how much they may buy or sell. However, should an insider make a very large buy or sell, it could attract scrutiny if he/she was in possession of material non-public information.
Generally, companies have blackout periods where insiders cannot initiate any transactions. An example of this is the period a few weeks before end of quarter up until public announcement of the quarterly earnings, where they are in possession of material non-public information.
The 10b5-1 plan allows the insider to have transactions take place at any time, including times where they normally would not be allowed to occur. In the example above, where transactions may be forbidden during the weeks before/after end of quarter, if a 10b5-1 plan is in place which specifies in advance the parameters which shares are to be bought/sold, then that takes precedence over the blackout which may be in effect, because it is generally done well in advance. The plans, when initiated, generally are in effect for a period of a year or more, or up until some fixed number of shares have been bought/sold.
Also, be aware, 10b5-1 plans are most always not filed with the SEC. You cannot go to sec.gov and search for insider 10b5-1 plans, where it is trivial to search for Form 4's. I have seen a handful of insiders attach their 10b5-1 plans to an 8-K filing for purposes of advertising it, so other investors would take notice. In the cases where I've seen this done, it was by unscrupulous insiders who were trying con investors, which eventually became clear.
So, an investor may not be aware that a 10b5-1 plan is in place if the insider or company has not made it known. Generally, if not made known by the insider when initiated, it will be specified in the Explanation of Responses section of the Form 4 when transactions take place. Filing of the Form 4 is always mandatory.
Here is an example:
Explanation of Responses:
1. This transaction was effected pursuant to a Rule 10b5-1 trading plan adopted by the reporting person.