It is not the job of the adc to kill the squishy priority champions
It is not the job of the adc to get kills
The job of the adc is to deal damage, nothing more, nothing less.
The adc deals damage and should not stop dealing damage.
The adc does not need to get the kills or secure them but doing so will make your life a lot easier most likely
If a squishy and tank are in reach you should aim for the squishy, not because it is the job of the adc or anyone in particular but because eliminating threats will increase your damage over time.
If a tank is nearby and a fighter can follow up or something and there are squishies too far away to be bothered with don't go faceplanting into the frontline to hit the squishies. It doesn't matter what the team wants to do, if they want to dive the backline and you are incapable of doing so you back off and let them die. If they need to dive they better have some shit about which adc they wanted in champion select. Vayne can get away with stuff like that, Ashe cannot. Jinx can as long as you don't fuck up. MF gives no shits, she's always ready to brawl. Cait kindof gives a shit. Corki would rather not but can. Trist will love you forever if you do it. Nobody cares what ezreal wants, make him do it.
Consider which adc youre playing and decide if your team should prioritize something and speak up before they screw you over but for the most part you'll just be hitting whatever is in reach not running in after something unless its really worth it.
A lot of adcs fail because they are afraid to counterbuild or attack a tank. "don't focus the tank" they say, this is some advice for noobs because noobs don't understand the longer version "don't focus the front line unless the backline is condition a,b,c,d, or e and take into consideration the prememorized build needs of the specific champ to determine if you must target the tank or simply yellow ping. If possible focus on helping to eliminate squishier threats and keep evasive maneuvers ready.. etcetc" Its easier to just say don't focus the tank but this isn't really good general advice for people who actually want to get better at the adc specifically or anyone who isn't an assassin really.
"hurrdurr don't focus the tank blitz!"
"but he was riding ashe like a pony!"