What things SQL Server takes backup of when we run a full backup

Like I am trying to ask that, suppose we run a full backup using GUI or T-SQL, which file SQL Server takes backup of i.e, only data file or log file as well.

I have read that it takes active transaction portion (Log File) of SQL Server as well. please confirm.

If yes, then what will happen to a transaction that was running at that time, and committed next, logs of the transaction will there on backup or not.

how will sql server revert back to it, if the database fails.