I don't really see the connection between your reprex and your question.
If you are more broadly interested in how to solve math probability questions, and answer those questions with simulations, I'd google around for examples and code vignettes on the topic.
With no assurance of quality, this appears to be a nice example with R,
http://math.slu.edu/~speegle/TempWebPageFolder/schedule/schedule.html
The Jan 20 and 22 lecture notes hit your topic.
For this specific question, the way you set it up I don't think you need to run simulations at all. If each team has a 50% chance of winning, and that's independent of whether or not one team won a previous match or not. So if a game is a coin toss, and the cavs winning is heads, it's like asking "what is the probability of getting 4 heads in 7 tosses, given the first toss landed heads". Or, what's hte prob of getting 3 heads in 6 tosses.
There are 36 ways to toss a coin 6 times. How many of those toss combos ends up with 3 or more heads?

6 fair coin flips: probability of exactly 3 heads
So you'd do this calc for (6,3) + (6,4) + (6,5) and (6,6), all the sets in which Heads was >= 3.
Here's a nice intro to probabilities with a 4-toss version of this question.
https://www.fourmilab.ch/rpkp/experiments/statistics.html