Hi @sns ,
First of all, if you are using the Modern experience of SharePoint you can't customize the master page nor the page layouts. Neither can you edit the CSS. You can do theming and site designs though. For custom snippets you can use SPFx to add custom extensions also web parts.
However for Classic experience, you can customize the master page and create your own custom page layouts. You can take a reference to this demo: Create A Custom Master Page In HTML For SharePoint Online. But, this is not a good practice and Microsoft will not provide support for this.
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