Mobile data is internet connectivity provided by your mobile phone provider. As you move around, you can receive both mobile phone calls and internet connections. It is mobile data that you are paying for and your SIM is instrumental in connecting you to EE for voice and (mobile) data as long as you have Mobile Data switched on..
WiFi is different technology. Unlike the network coverage provided by your mobile phone provider, WiFi depends on a router in a fixed place. WiFi provision is provided by businesses, shops, homes - they pay for an internet connection terminating on a router. As you move around, your phone may see many WiFi SSIDs (network Service Set Identifier - the name of the available network). You need to set up a connection to WiFi on your mobile phone. Usually you will find a password provided by the business offering the connection.
Warning - free Wifi is convenient but comes with significant risks, it is hard to stop hackers intercepting messages. If you know what a VPN is, you should turn it on. If you can't encrypt your messages, make sure you don't access any sensitive data while on a WiFi connection that you have no control over.
Mobile phone companies usually enable WiFi calling - that is you are able to make phone calls from your mobile using your home WiFi. This is useful where a mobile phone signal is poor.