Protobuf uses a bunch of variable length encodings. Here it's decoding a TLV format, but the length is itself a variable length integer (and seems to be a kind

Protobuf uses a bunch of variable length encodings. Here it's decoding a TLV format, but the length is itself a variable length integer (and seems to be a kind of tag-value encoding?) where you basically get 7 bits per byte telling you the value, and the leftmost bit tells you whether there's another byte. So if you mask with 0x8080 and get zero, then it was a 1 byte (7 bit) integer. If 0x8080 is not set, then the tag-value record is 2 bytes. Left byte has tag. Right is value. Then they're masking with 0x0700 to get the type of record, which should be LEN. So if it's a single byte LEN record, they can take that single byte as the length (they mask with 0x00ff, but really it's 0x007f. They already know the 0x80 bit is zero, and the value is contained in the least significant 7 bits). Otherwise they have to do some fiddly logic to decode the variable length integer to figure out the length (length here being the L in TLV).