Online sources, except patents, should include a link back to that source. But as you have no doubt seen: links to pages and entire websites break.
A lot of scholarly websites will have a DOI for each object. DOI's are most associated with journal articles.
For the DOI https://doi.org/10.1021/jacs.1c11434, the 10.1021 is for the publisher (American Chemical Society), while the jacs.1c11434 is the unique identifier for this JACS article.
Digital Object Identifiers are persistent. If the URL for the object changes, you should be redirected to the new, correct URL.
If there is no DOI, use a URL. Test the URL in another browser to make sure it really works. Simply copying and pasting the URL from the address bar will not work on many sites, including some databases. If the site offers a recommended URL, start with that. If you cannot create a stable URL for the citation, use the site's homepage URL.
For most sources with a URL, include an access date at the end for when you used the source. If the source disappears, the date may help someone if the URL no longer works and they want to see if it was archived for the Wayback Machine. The new style guide recommends ISO 8601 standard of year-month-day (2021-05-06), but the more common "month, day, year" should be fine.
