Previously ordinary BLAKE2b-256 was used, but it prevented partial calculations of file parts, so you had to fully read the whole file again after its resumed download.
MTH divides data on 128 KiB blocks, hashes each of them independently and then calculates the Merkle tree root:
node3 / \ / \ node2 leaf4 / \ \ / \ \ / \ \ / \ \ / \ \ node0 node1 leaf4 / \ / \ \ / \ / \ \ leaf0 leaf1 leaf2 leaf3 leaf4 | | | | | block block block block block
Leaf’s value is keyed BLAKE3-256 hash of underlying block (128 KiB, except for probably the last one). Node’s value is keyed BLAKE3-256 hash of two underlying leafs. Keys are BLAKE3-256("NNCP MTH LEAF") and BLAKE3-256("NNCP MTH NODE"). Keyed operation allows working with an aligned data (128KiB or 64B boundaries), unlike popular way of prepending 0x00 and 0x01 to the hashed data, being more efficient with an attention to BLAKE3’s internal Merkle tree.