I used to work in boarding but have never been a guardian myself.
Guardians could be called upon to come to school for any situation that parents would usually be asked to come for (e.g. meetings over serious matters, if the child was very sick or injured and had to be removed from the school, or if the child was suspended). Guardians were also expected to take the child for exeat and half-terms as well as for any gaps between the start/end of term and flights to/from home.
Guardians were not required, but were encouraged, to attend special events at the school like plays and prize givings.
Things like parent-teacher conferences usually took place over the phone/video conferencing with the parents and not the guardian.
The guardian was not required to pay school fees, uniform fees, or activity fees if the parents did not pay.
With the child coming from Ukraine, I would be concerned that they may be unable to go home at all (e.g. you would look after them for Easter, Christmas, and summer).
In most cases being a guardian was not onerous and was more of a formality,