The open-source IllumOS code in FreeBSD had fallen behind Linux ZoL (ZFS-On-Linux). FreeBSD adopted ZoL to keep up with new bugfixes, enhancements, etc.
>>The OpenZFS project brings together developers from the Linux, FreeBSD, illumos, MacOS, and Windows platforms. OpenZFS is supported by a wide range of companies.
It remains a fact that operating a coal plant costs substantially more than getting the same power from wind.
So, continued operation of coal plants depends on uneconomic behavior, typically via cultural inertia or domination by entrenched interests. Of course that is always possible, but it is disfavored.
As suggested, one path is to port Rockbox firmware for your MP3 player. However, since you do not have any prior experience in embedded domain, it will not be a easy path.
I would suggest that you try using the Arduino or RaspberryPI hardware. Both of these platforms are widely used, so you would be able to easily get help if you get stuck. Also, these are 'simple' platforms that are widely available, so the learning materials available is unmatched.
Building a simple project like a weather station or clock will keep you focused on a "actual" target/goal.
In India, you'll typically buy your phone outright based on what features you're looking for (Dual SIM, OS version, Storage, etc). Hence manufacturers/retailers typically offer a EMI plan for top-end phones (think Samsung S7, Apple iPhone 6, etc) since the outright cost is quite high.
Then you'll decide which carrier you want to use based on the tariffs they're offering (Voice Calls, data, SMS, etc.)
Although carriers do offer phones with their plans, it's rarely ever bought in that manner.
https://www.freebsdnews.com/2019/01/10/zfs-on-freebsd-zof-is...
So, for open-source, ZoL is the 'mainstream' ZFS.