In its 2022-03-18 GOP lawmaker slams Trump's Ukraine remark: 'He was awful' "CNN's Jim Sciutto talks to Rep. Adam Kinzinger (R-IL) about Donald Trump's comments about Putin's invasion of Ukraine". At the end, Sciutto asks the US Representative:

Final question if I can. You are among the names mentioned who might challenge (Trump) for the nomination in 2024 should he decide to run (and) it looks like he will, along with Liz Cheney perhaps Mike Pence. Do you believe there will be a competitive GOP primary, that you will get the opportunity to do so? We saw steps in 2020 where the GOP closed the path to challenges. Do you believe that will happen again? Or will you get a shot?

That Kinzinger did not challenge the premise of the question in any way lead me to suspect that there might be some demonstrable truth to it, and to wonder if there are still ways that the GOP could "closed the path to challenges" to former president Trump should he run in 2024.

Question: What steps could the US GOP take to block primary challenges to Trump in 2024 should he run again, and which of these were also taken in 2020?

note 1: It is possible that some steps in 2019 have been discussed in previous questions tagged republican-primary and it will be helpful and informative to include links to those where possible.

note 2: fwiw Sciutto uses "closed the path to challenges" rather than challengers.