You can refer to the left roadmap in the following article to start with SharePoint online.
Introduction to SharePoint in Microsoft 365 for your reference:
https://learn.microsoft.com/en-us/sharepoint/introduction
Microsoft announced that they had plans to officially retire InfoPath 2013 after this version. Microsoft is still maintaining support of InfoPath 2013 until 2023.
You can using PowerApps instead of InfoPath and Microsoft Flow for SharePoint Designer.
For your reference:
http://www.formotus.com/16643/blog-infopath/microsoft-announces-future-infopath
https://www.jijitechnologies.com/blogs/powerapps-forms-for-spo-list-infopath-replacement
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