AD&D 2e ( the last time I played PnP ) was a more loosely defined experience in many ways, and much was left up to the DM to decide how things flowed best in any situation.
Turns were approximately 10 minutes, used while travelling, searching and other out of combat activities. Combat used approximately 1 minute rounds, during which you could do only one activity. So you could attack with a weapon ( or grapple, wrestle, overbear, punch etc ), OR cast a spell, OR bind wounds, OR drink a potion etc.
Activities like attacking were assumed to include movement ( up to half movement points ) plus maneuvering/feinting etc, not just a single blow. Some high level characters could roll for hit 4 or 5 hits in a single round.
Activities like casting a spell, or drinking a potion were assumed to involve retrieving items from wherever they were on your body and preparing them, while under combat stress.
Basic initiative for turn order was rolling a die for each side with modifiers for surprise etc. Tied initiative was ALLOWED, meaning 2 combatants could kill each other. There were advanced initiative rules that were optional, for which the combat round was divided into 10 segments of 6 seconds each. Both spells and weapons had a number from 0-9 associated with them ( which indicated how long the spell took to cast, or how slow the weapon was ) which indicated the segment in which the spell fired or the weapon first hit.
In real PnP play, the DM would probably allow more to occur in a round than this, because pulling a potion from your belt pouch and drinking it is probably NOT a 60 second action. One common optional rule was to allow more than one spell if their combined casting time ( in segments ) was less than 10.
Because the rules of 2e were a lot looser than later editions ( and ultimately up to the DM's discretion ), it was easy for the developers of BG1/2 to build a system that suited them.
The 60-second round made up of 10 x 6-second segments, has since changed into the 6 second combat rounds familiar from 5e. It is interesting to note in the BG3 gameplay, just how much a character can do - move, cast a spell, drink a potion and remove your boots to throw at/to someone, all in 6 seconds. Impressive!
