for "true / false" part:

=IF(ISTEXT(REGEXEXTRACT(A1;"sites"));"True";"False")

for "more words crossreferenced with another sheet" part:

=IF(ISTEXT(FILTER({'sheet1'!A1:A3};{'sheet1'!A1:A3}=A1));"True";"False")

You can combine FIND or SEARCH (which is case insensitive) into LARGE and ARRAYFORMULA functions:

=ARRAYFORMULA(ADDRESS(LARGE(ISNUMBER(SEARCH(G1,A1:F15))*ROW(A1:F15),1),LARGE(ISNUMBER(SEARCH(G1,A1:F15))*COLUMN(A1:F15),1)))

If search string is not unique in range, it will find last cell that contains string. If you want find first cell, change LARGE to SMALL function.

Short answer

Use find & replace

Explanation

1. Press Ctrl + H to open the Advanced Find and Replace dialog
2. Write A5 on the Find field
3. Turn on the Also check within formulas option. This will turn on the View All formulas spreadsheet option.
4. Click the Find button

Tips

Use \$?A\$?5 and check Search using regular expressions