They offer a one time purchase for “premium” (I don’t remember the price because I purchased ages ago using play store credit I got from surveys mainly to support the devs). Here’s what the documentation says about it:
Free edition The free edition only has a single limitation, a maximum of 30 running blocks, that could be 15 running flows with two blocks, or a single running flow with 30 blocks. Unused/unconnected blocks within a flow are also counted. There’s no limit on inactive flows stored on the device. All features can be used with or without Premium purchase (if supported by device).
How it’s living rent free in the head of many lemmy users who keep making posts aboit it