I recommend listening to the Move the Sticks podcast to get a better idea of how teams evaluate, grade, and stack their boards.
Players fall into a range based on team grades. Most drafts will only have 15-20 players who grade out as true first rounders. Few will grade out to be top prospects at their position (blues) and those mostly go in the top 5-10.
The decision to trade back is based on how many players you have available in that range when you pick. If you have 5 players on your board at pick 9, you can trade back to 13 and be certain one of those players will be there (4 players will be picked and you’re okay with any of the 5).
The thing most people don’t get is another team has to be willing to trade up. Reaches can be situational. Sometimes no one is willing to trade with you and you’re forced to overdraft a player.
Lastly if you have conviction on a player, especially at a premium position that lines up with need, it won’t matter how many players are there. It’s an easy decision to run the card in if a top 5 talent drops to 9.