blazer is component based html render engine. it about responding to browser events, and modifying the html. it is not tied to bootstrap, just the sample templates use bootstrap because its popular. newer templates typically use less bootstrap.
the docs cover authentication and authorization:
https://learn.microsoft.com/en-us/aspnet/core/blazor/security/?view=aspnetcore-7.0
bootstrap is one of many css frameworks. materialize (android style), tailwind, semantic ui, and office ui fabric core (for an office 365/online look).
https://materializecss.com
https://tailwindcss.com
https://semantic-ui.com
https://github.com/OfficeDev/office-ui-fabric-core
once you pick a CSS framework, you will find lots of documentation on using the frameworks. Most are now SASS based, and require some html/css expertise.