By number. With a patent number or patent application number, finding the document should be not too hard. It should be just a matter of finding a database with the right content and matching the required format. Searching Espacenetis a good option. Doing a regular Google search with the title or patent number is another good way to search.
Many (probably most) patent documents are available to the public. Remember that:
As legal documents, they have numbering conventions and special abbreviations
Regular Google may be fine for locating the item you need
Sometimes you will need to go to --- or it will be more straightforward to go to --- a patent database.
The European Patent Office provides a country code table to help with forming a search or understanding a patent's origin. For example:
"US" designates a document from the United States
"MX" is for Mexico
"DE" is for Germany.
We might expect to see any of these formats for the same granted (issued) U.S. patent, depending upon the person doing the citing or upon the database. Be sure to check on which is required --- especially if you try one that gives "0" results.
As with the record for patent US9796219, Espacenet may show other numbers besides the one you searched. In fact, the record retrieved with the search is US2017136834 (A1). US9796219 (B2) is in the "Also published field."