Ethernet Frame Structure

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• What's the purpose of the preamble?

It's out of 8 Zeros and indicates the start of a frame. So the receiving station knows, where the very first Bit of the Frame is.
After this, there comes the Frame Delimiter (SFD)

• How many Bits are used for the MAC-address?

Each MAC-address (Destination- Source-MAC-address) needs 6Bytes (48Bits).

• Where is the IPv4/IPv6 address hidden?

Inside the Destination & Source Address.

• How does the receiver distinguish between Ethernet and 802.3 frames? (Difference between type/length field)

At the IEEE (Institute of Electrical and Electronics Engineers) 802.3, instead of a "Type"-Field, there is a "Length"-Field.

• Which algorithm is used for calculating the frame check sequence?

There is inside a checksum (32 Bit). The Frame Check Sequence is going to be created at the transmitter. The recipient also calculates a checksum and compares it to the checksum of the transmitter.
If they don't match, the datablock is going to be deleted.