Boundary changes across tract 5-year estimates

Hi, Can anyone point me to information about how ACS handles Census Tract boundaries for 5-year estimates when there's a boundary change within the 5 years. (i.e. a 2021 estimate has 3 years that would be using 2010 Census boundaries, and 2 years using 2020 Census boundaries). We know that these 5-year estimates are released in 2020 boundaries, but we're unsure how ACS apportions the pre-2020 data into the 2020 Census Tract boundaries. Thanks!