An internal poll, while it might be rigged in favor of a candidate, but it doesn't make sense for a poll to be rigged in favor of a candidate. If a poll is biased/rigged then it doesn't give the candidate an accurate representation of the electorate, and wouldn't help them at all, and also mislead them.
Obviously the candidates try to get accurate data for their own campaigns. Usually, none of that data is released to the public. When campaigns do release polls it's to push a narrative, and they have every incentive to mess with the data.
Yes, candidates are often encouraged to publish bullish poll numbers to attract attention and/or energy to their campaigns.
Clear favorites will also sometimes release polls that show things competative in order for fundraising.
