I don’t understand why I can watch the first few episodes & then pay

Because that's the offer.

You get 7 days free and seasons 1-3 to watch in that time. Past the 7 days and for season 4 you need to be subscribed to the History Vault service to continue watching or buy the season or episodes from the iTunes Store.

They are not entirely free.

You must have watched on some other channel or App within the Apple TV App It’s on Prime as well as other channels or Apps but I’m not sure if it’s free or was free on those.. It’s certainly not a show I can watch for free on my Apple TV+ subscription - it asked me to pay. Do a search for it and scroll down to “How to Watch” to see if it’s offered elsewhere for free. Or it may be on YouTube. Tubi has the Skinwalker Ranch movie for free. That App is absolutely free.

It does t look like Skinwalker Ranch is included in the Apple TV + subscription. It’s not an Apple Original.

SJSooze wrote:

I was able to watch season 1, 2, 3 and episodes one through five (I think) of season four. After that it was &2.99 an episode. I just didn’t understand why that is.

All of the Apple TV content is free and the other Channels may have offers for a free trial that is not controlled by Apple. Skinwalker Ranch is the History Channel and they are the ones that choose what content you can view for free or how long of a free trial you could get. Each channel will have a different offer.