In Classic SharePoint Online Sites, If Scripting capabilities are enabled then We can Autopopulate Values Like Firstname, Lastname etc using below options
1.Based on PeoplePicker field value in other fields using JQuery (Script Editor Webpart)
2.Using SPServices and JQuery to autopopulate values from UserProfile.(Script Editor Webpart)
To Achieve Same Autopopulation Logic in Modern Team Sites. What are the options going forward
1. since Using SPFX we can interact with Page and Webparts but not with NewItem Listform Fields and UI .
2. Is PowerApps the only option Where we can autopopulate values using functions based on Peoplepicker field Datacard Values (But Forms designed using Powerapps require all user to have PowerApps Subscription)