You can divide transformers into two general classes, 'autotransformers' and 'isolating transformers'.
In an isolating transformer you have separate primary and secondary coils. There is no continuous electrical path from input to output. With an isolating transformer _all_ the transferred power goes through the magnetic core. You also get complete control of secondary grounding.
In an autotransformer you do have a connection from primary to secondary. Only some power passes through the magnetic core, some goes directly from input to output. The grounding remains unchanged from input to output.
A 'buck/boost' transformer is an autotransformer. Going from 208 to 220, 1kVA of transformer can support 18 kVA of loads. This is why people use autotransformer setups. But you don't get a balanced 110/220V grounded neutral setup. You get some odd unbalanced single phase where one leg is 120V to ground, the other leg is >130V to ground, and you don't have a neutral.
You could feed a subpanel with a buck/boost transformer, but you shouldn't unless the odd voltage and lack of neutral is fine for the loads being served. You might do this if you are using a 3 phase buck/boost arrangement to feed pure 3 phase loads.
When an isolating transformer is used to feed a panel, the common setup is to use the panel main OCPD as the transformer secondary OCPD, and to size the secondary conductors to follow tap rules.
If the panel is too far away to meet the tap rules, then you will need a local OCPD.
If you have a pure 208:220V single phase transformer, then the primary can protect the secondary. But if the secondary is split with a neutral tap, then you must have secondary protection.
Jon
