There is no free official Google Trends API, and that sentence saves most readers a long search. Google's own Trends API is an alpha granted by application. The free routes that do exist are the Trends website by hand, the pytrends library against that website, and for approved projects the alpha itself. Each is free in money and priced in something else. This page ranks them by what they actually cost, then points at the managed route for jobs that recur. The key question is answered on Google Trends API key.
Route one: the website, by hand
The Trends website costs nothing and returns charts and CSV downloads. It is also manual. A person opens the page, sets the range, and exports. That works for one question and fails as a pipeline: no scheduler runs a browser habit, and the download is a file, not a dated series a system can store.
Route two: pytrends, free and finished
pytrends wraps the website and made a generation of notebooks possible. GitHub marks the repository archived. Calls meet rate limits that surface as TooManyRequestsError, covered on pytrends 429 errors, and the repo status sits on pytrends is archived. Free, and fine for a prototype. Expensive as infrastructure.
Route three: the alpha, free and gated
Approved alpha projects pay nothing and wait instead. The application decides, the window is five rolling years, and the values are not the website's 0-100 scale. A team that can wait and needs Google only may land here happily.
The managed route
Trends API sells the boring parts: one POST at https://api.trendsapi.ai/api, a Bearer key, weekly points on a 0-100 scale, and growth over named windows. The same key reads 16 keyword sources, so the second platform is a string change. Terms and any starter allowance sit on pricing. The endpoint tour sits on Google Trends API.