For Science Direct, the URL in your browser's address bar is stable and can be used to link to an article.
However, these links can be long and you may wish to use the neater-looking DOI URL below the article's title and authors. If you do it this way, you must copy the link location, rather than copying the URL as displayed. On a PC, you can do this by right-clicking and selecting "copy link location" (the exact phrasing may vary with different browsers).
In the image above, the DOI URL is https://doi.org/10.1016/j.vaccine.2020.01.080. Using "Copy Link Location" results in this URL: https://doi-org.ezproxy.delhi.edu/10.1016/j.vaccine.2020.01.080. The "ezproxy.delhi.edu" component is essential for prompting students to log in and access the article through the library's subscription. Without this, they will not be able to access the full text.