Tuning typically follows a little ritual, where (1) the concertmaster stands to indicate that tuning should begin, (2) the oboe gives the 440 Hz A tone, which by the way, they normally have an electronic tuner these days to help them nail that pitch, (3) the brass and winds tune off the oboe, (4) the concertmaster tunes her violin off the oboe, (5) the concertmaster plays the A for the strings, who tune their open A string off that note, (6) the strings tune their other three strings to their A, usually by ear.

Source: Me, playing in amateur orchestras for twenty-plus years.

Sidenote: I find the sound of an orchestra tuning to be a deep joy! All those instruments sliding into place is just delightful