Questions Regarding internals of Block and Index

I am new to InfluxDB,
got few questions about blocks in TSM file and inmem index.
From comments in writer.go I found block contains data with CRC but how that data is represented is not specified
is it like <series_id_1><series_id_2>…<field_value_1_1><field_value_1_2>… <field_value_2_1><field_value_2_2>… ?
From (InfluxDB Internals 101 - Part Two | InfluxData)
One thing was not clear to me is that can we directly go to a specific block using this inmem index or we can only go to specific shard and then search all of that shard??
And what does KEY in index of the tsm file corresponds to is it series id??