As an English word, campus is a relative newcomer: it is first attested in the United States of 1774. From the original Latin campus, Middle English had only the French derivative champ, which was still used by Elizabethans in phrases like “the champ of battle.” Its link with champion is fairly obvious, but you may not easily see the connection with French champagne and champignon (“mushroom”—a delicacy that grows on the open field). In 1528 the word camp was borrowed (via French) from Italian campo; similarly, campaign (a doublet of champagne) came through French from Italian campagna (from Medieval Latin campania). Except in Norman dialect, Latin ca– always developed into French cha-, unless there was some other form of foreign influence. It is somewhat ironic that campus has recently made its way into French as an English loan-word.
Latin populus forms the P in the celebrated acronym SPQR (senatus populusque Romanus, “the Senate and the Roman people”). In French it became peuple, source of English people. The corresponding Italian form is popolo; by means of an expressive pejorative suffix, popolo was changed to popolaccio (“the bad people”), and English got the word populace. The Spanish derivative of populus, pueblo, came to mean a town or village, and is so used today in English.
There is a small sub-type of the 2nd declension –us noun where the nominative singular (the vocabulary form) ends in –er. Minister and arbiter are English words that have kept their original Latin form. Another Latin noun of this type was magister, “teacher,” “(school-)master.” Through Old French maiestre this word became master; the modern French maître is also used in English as maître d’hôtel or maître d’. Doublets of master are mister, Mr., and maestro, the Italian derivative. A 1st declension feminine counterpart, magistra, became Old French maiestresse, whence English mistress, Mrs., Miss, and Ms.