obesity AND diabetes mellitus, type 2 AND bariatric surgery would be terms you would want to search.
All 3 of these are MeSH terms. MeSH is used by the National Library of Medicine to index articles and other information.
MeSH database is located under Explore on the PubMed site
Limit to appropriate study design
This is a therapy question. We know from the previous discussion that the best evidence for a therapy question is a randomized controlled clinical trial (RCT). Use the Filters column from the main results page to limit to Randomized Controlled Trial as an article type. You may need to click on "more" to see additional filters if RCT is not listed.
some filters are on the left side of the screen in PubMed (including RCT)
You can also use the Clinical Queries for this search instead, this is under Find on the PubMed homepage, but it will take you to a page like below
