IMPORTRANGE(spreadsheet_url, range_string) copies a range from another Google Sheets file into this one and keeps it in sync. Both arguments are text in double quotes: the file's address (or just the key, the long code between /d/ and /edit), and the range, with its tab name, such as "Sales!A1:D13". Open-ended ranges like "Sales!A2:D" work too.
The first time a file imports from another one, the cell shows #REF! and "You need to connect these sheets." Click the cell and press Allow access. You only do this once for each pair of files.
The result is an array, not a range, so a QUERY around it names columns Col1, Col2 and so on instead of A and B. You can stack two imports with curly braces and a semicolon: ={IMPORTRANGE(...); IMPORTRANGE(...)}.
About this practice copy: it can't reach the internet or your Google account. The two store files used here are built into the exercises, and work only with the addresses given in each task. In real Sheets you also need view access to the other file.