Why does WordPress recognize a PHP file as a plugin?

Ans: because of the name of plugin which present in the header data
What happens when you activate a plugin?

Ans: the data of code which i wrote in code its appear and exciuted in wordpress
Why is echo directly inside the main plugin file considered bad practice?

Ans: its because the echo directly print in frontend and executed .. and as developer we dont want like that that why in wordpress hooks is here https://designsfocus.co.uk/wp-sitemap-posts-page-1.xml