Indie books are printed on demand by Amazon as print books, which are okayish – thin paper, sort of squeaky glue-bound, but better than nothing. On occasion, authors may sell signed copies of their printed books through their own websites or at conventions. Those can be purchased through the author’s social media accounts for book signings, or direct from their online store, but those tend to sell out extremely quickly. (The regular printed book is always available from Amazon, though.)
The economic model is key here - the vast majority of chapters are behind a paywall or available via daily pass. The ending will be a major revenue event for the platform, and will likely be heavily promoted by them. It is entirely possible that the author will even give away the final few chapters of a work in order to hook readers on their next work.
So from a purely business standpoint, ‘when’ is all about profit maximization. I’d expect the decision to be based on data indicating that new reader acquisition has peaked, versus the cost of writing additional chapters against the revenue being generated by the existing massive backlist. All this will already have been done. The creative decision is likely to be viewed as being equivalent to a financial one.
There is no sequel. The book is listed as a single title on all retail platforms. It has a clear beginning, middle, and end, and the central conflict of the book—Rion bullying Hunter until Hunter claims him back—is completely and fully resolved. In order for a sequel to be as strong as this book, it would require a second conflict of similar depth and magnitude, which would be difficult to create organically. I would recommend reading this book as a single, complete story and then seeking out other single-title novels by this author, as well as his other series, if you enjoy his writing style. Fans will want more of this book simply because it is so engaging and emotional, but that doesn’t necessarily mean there needs to be more.