Making a calculator = creating schematics and PCB for electronics, and then making the software
Over the years, there have been multiple projects aiming at creating DIY calculators, on multiple message boards of the community, including this one. Some of the message boards do not longer exist / are no longer active.
One of the newest and most advanced such projects is the Symbolibre:
https://symbolibre.org/en/ . The activity has lowered over the past few months.
There's also a commercial calculator whose mechanical and electrical schematics are publicly available:
https://www.numworks.com/resources/engineering/hardware/ . Neither the N0100, nor the recent N0110 have more RAM than a ~1998 92+ to 2004 89T (TI-68k calculator series) or a 2015+ 84+CE / 83PCE / 83PCE EP; however:
* their CPUs are in the high end of the spectrum for a calculator, especially the N0110's;
* the N0110 has a built-in 8 MB NOR Flash memory chip, and the same chip (or better, a 16 MB chip) can easily be soldered on the N0100's PCB. That's more than enough to fit a high-featured build of the powerful giac, which underlies XCAS for computers, the HP Prime, the Symbolibre and KhiCAS for a range of models including the TI-Nspire and the Casio fx-CG50 / Graph 90+E (due to size limitations, KhiCAS on the latter has crippled functionality) - the bottleneck is the low amount of RAM, which allows for simple (enough) computations but prevents the most complex ones.
Of course, the Symbolibre and NumWorks calculators natively provide programming languages other than TI-eZ80 TI-Basic, though the Symbolibre is powerful enough to emulate a 84+CE / 83PCE / 83PCE EP.