>>2917867
There are analog multiplexers like the CD4051 or switches like CD4066 but it become impractical once you have many cells, but since many microcontrollers have built-in multiplexers in their ADC inputs it's easier to just chain a bunch of micocontrollers all connected to an insulated digital bus, like tesla and many others do in their battery cells

See https://github.com/stuartpittaway/diyBMSv4 for an open-source version where each module controls a single cell

>>2917871
There's plenty of chinese active balancers that can balance up to 8A, but it's best to just test indivudually each cell and spread them evenly in the pack