An audio or video source is the initial component of the sequence.  Audio and video sources include compact disc players, turntables, DVD players, BluRay players, cable boxes, satellite receivers, Internet streaming devices, AM/FM tuners, audio cassette decks, game consoles, off-air television tuners, Bluetooth devices, etc. 

Each source has output jacks that feed the input jacks of the next step in the sequence via -- HDMI, single-ended RCA, optical Toslink, or balanced XLR -- interconnecting cable.  Sources with Bluetooth and WiFi transmitters connect to Bluetooth and WiFi receivers of the next sequence step.

The next step in the sequence path is the audio preamp or audio/video preamp.  Sources enter the preamp via dedicated labeled input jacks or wireless Bluetooth and WiFi receivers.  The preamp switches between connected sources via a knob or row of buttons on the preamp front panel.

Prior to the exiting the output jacks, selected sources are manipulated and or processed via another knob, buttons, or the setup menu. 
Manipulation includes volume, bass/treble control, equalization.  Processing includes surround formats, digital audio formats, video scaling, video trans-coding. 
The selected source is then routed to the next step in the sequence.

The audio preamp output jacks connect to the input jacks of the amplifier. 
The amplified audio then continues to the speaker connecting output jacks.

The AV preamp video output connects to the video input of a TV or projector. 
Video amplification is integrated with the TV video decoding circuitry.

The amplifier output speaker jacks are connected to the speaker input jacks.
The modulating speaker cone reproduces the ultimate sound waves.

The AV preamp output connects (typically via an HDMI interconnect) to the TV or projector input jack.
The combined output of the TV video decoder/amplifier illuminates the pixel elements of the TV or projector.

1. TV Confusion
The television can add confusion.  TVs include additional input jacks plus "The Sequence" of an off-air-TV-tuner source > switching pre-amp > amplifier > speaker and TV screen.  This permits the TV to perform as a stand-alone low-fidelity audio plus video system.  

2. *Pre-amp Adjustment
Many audio pre-amps and receivers have eliminated the phono preamp stage.  If a turntable is in play, then a dedicated-phono-pre-amp must be added and inserted to the input of the pre-amp.  The phono pre-amp boosts phono cartridge voltage and provides RIAA specified playback equalization.

3. Sequence Note
A stereo or AV receiver is a source AM/FM tuner + pre-amp + amplifier in one cabinet.
An integrated amplifier is a pre-amp + amplifier in one cabinet.
A portable radio is an AM/FM tuner + pre-amp + amplifier + speaker in one very small cabinet.

4. Video Sequence Route Note
The video source determines the interconnect path to the TV.  Contemporary product is widely limited to HDMI cabling.  However, legacy product may employ composite video, component video, or RGB interconnects.  If you start a sequence with composite video, then composite cabling must continue to the TV.  If component cable, then component cable must complete the sequence.

Multiple types of video cabling requires multiple hookup paths to the TV, plus video source switching, unless the receiver/preamp includes video transcoding. 
Typical transcoding converts composite and component video to HDMI.   However, transcoding has disappeared from many current AV receivers.  Therefore, ask customers if they plan to use legacy video sources.  Then choose the appropriate receiver/preamp.

5. Interconnect Jack Note
Each sequence step above described connection via outputs and inputs jacks.  Integrated devices (as in Note #3) are connected on their PC boards.