I am new to the SPFx framework, I do not know how to use
SPFx in modern site page.
How to customize the navigation bar in a modern site page and how to increase the navigation bar?
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You can customize here in the "Global Navigation" section.
You don't need SPFX for this.
You can learn about SPFX from the samples
You need to use SharePoint Framework(SPFx) Extension to extension to achieve that. Follow this MS doc for more: https://docs.microsoft.com/en-us/sharepoint/dev/spfx/extensions/overview-extensions
Just remember that the changes made through SPFx extension will be reflected ONLY on modern pages, not the classic pages(e.g: Site Contents, Site Settings).