Calling "Bot" at champ select

This is just an easy question I haven't been able to get an answer to yet. I haven't played for about a year so I'm still getting caught up: In champ select people call "BOT" as a position then go something like Azir / Yasuo / Ahri and I'm not sure what I should do as the second person heading bottom. Do I support or play ADC or something else? I watched a little LCS and they were talking about how ADCs aren't used as much anymore so what does "BOT" mean now and what should I do when my lane mate calls bot? My guess is go sup and buy support items? I've been playing LoL on and off for a while now and as far as I knew a class marked as a Marksman (ADC) + one marked as support is what they wanted in the bottom lane.
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