I want to search all keywords, documents, list values, etc everything within a Site Collection. Which out of box search should I use for it, and how should I configure the search.
Any help is appreciated.
As a SharePoint Online administrator, you can customize the search experience for users. This customization includes defining searchable managed properties in the search schema, identifying high-quality pages to improve relevance, managing query rules and result sources, and removing individual results. You can also evaluate any changes by viewing reports about usage and search.
The changes you make from the search administration page are valid for the whole tenant, but you can also customize search on site collection level and on site level.
The Below links will be helpful for configuring Search in your environment.
SharePoint online Search Administration OverView