There is no specific way to download a SI from a journal. Usually, it will be at the bottom and easily available to download. In some cases, you may need a subscription to download SI also.
or ask someone who can access the journal.
If nothing working for you, the best way is to contact the author. They definitely send you the paper and SI.
I confirm the previous answer: you can download supplementary material by following the link provided at the end of the online or paper article, but generally only if you have a subscribed access to the article.
To get ESI/supplementary information of Elsevier journals, you can get instruction to authors who wish paper publication .If you open an issue of the journal first or last page of the journal.