(Note: this is not a request for help with "homework"; I'm 51 years old and am trying to learn on my own, through my own "assignments" to myself.)

I'm pretty new to / weak in statistics and am trying to learn more about probability. I have built a simple coin-flip simulator in Excel using VBA. It uses this formula:

=IF(RAND()<0.5, "Heads", "Tails"), where the RAND function generates a random decimal between 0 and 1 every time the sheet recalculates.

I'm experimenting with up to 20,000 flips per simulation. Judging by the results, it seems to correctly simulate the tosses of a fair coin, because as the number of tosses increases, the Heads and Tails results as a portion of the total each approach 50%.

I am observing what kind of "streaks" I get. By "streaks" I mean consecutive, identical outcomes. For example, a Heads streak of 5 means I get Heads 5 times in a row.

I understand (or think I do ; please correct me if I am wrong), that if you flip a fair coin, you calculate the probability of a given streak as (1/2)^streak size. For example, the probability of getting Tails 3 times in a row is (1/2)^3 = 1/8, or 12.5%; the probability of getting Tails 4 times in a row is (1/2)^4 = 1/16, or 6.25%; etc.

That's all fine ( I hope). But...

-- let's say that after a simulation I observe, among other things, that I get 4 separate streaks of 7 consecutive Heads. The chance of getting one streak of 7 consecutive Heads is 1/128, or 0.78125%; but what is the chance of getting 4 separate streaks of 7 consecutive Heads?

-- suppose further that I get, in the same simulation, not only 4 separate streaks of 7 consecutive Heads, but also 2 separate streaks of 7 consecutive Tails? How do I get calculate the probability of all this happening?

I'd be grateful for any layman-friendly advice!