uzi_hs The bela headers - the pinout is not like the BeagleBone Black?
It is. All the pins are a pass-through from the BBB. Some pins are used on the Bela cape, most are not.
uzi_hs In Bela documentation it’s written that P24 and P26 are SCL and SDA, and in the Black documentation it seems that these pins are UART1.
Each of the BBB's pins can assume one of several functions and the same function can be associated with more than one pin. So yes P9.24 and P9.26 can work as UART1 and are named after that, but on Bela they are set to I2C1_SCL and I2C1_SDA, respectively.
uzi_hs In general - all BeagleBone Black pins are available to be used according to the documentation that I find online?
Some pins are used on the Bela board, some others are used on the BeagleBone itself, some of which have special meaning on the BBB at boot. See here for more details and in particular the pinouts https://github.com/BelaPlatform/Bela/wiki/BeagleboneBlackBelaP8HeaderTable.pdf and https://github.com/BelaPlatform/Bela/wiki/BeagleboneBlackBelaP9HeaderTable.pdf, where the current function of the pins used by Bela (and I2C1) is highlighted.