EDIT: If you are too lazy, the first link is about a polling company getting paid by Priorities USA (Clinton Organization) and oversampling Dems.
Firstly, it's a simple fact that there are more democrats than republicans, and by a pretty substantial margin as well:https://en.wikipedia.org/wiki/Democratic_Party_(United_States
To be exact there are 41,341,965 registered democrats, compared to just 30,447,217 republicans, or a margin of 11 million voters. The last time an election was settled by more than this was 1984, when reagan won by 16 million votes. Since then, no candidate has won by more than 9 million, which makes this 11 million voter discrepancy a pretty big deal.
Also, The sections you see in polls about partisan makeup are literally just the polling company asking someone "Are you a democrat or republican?". While some people definitely do feel a strong attachment to their party, a lot of people don't and just say whatever they feel like. SO for example if there is, say a controversial video about donald trump going around, some people will identify as democrats rather than republicans. Conversely, if wikileaks drops some leaks on hillary clinton, some people will probably say they're republicans rather than democrats.
Want to talk about rigged, go look at the 2000 election.
Not sure what you're talking about, please clarify.