He assumed this office with zero democratic process

Quoting this site:

Article 233 of the Venezuelan constitution states that “when the president-elect is absolutely absent before taking office, a new election shall take place (...) And while the president is elected and takes office, the interim president shall be the president of the National Assembly.”

Antonio Ecarri, a constitutional lawyer and vice president of the opposition party Democratic Action (Accion Democratica) said that article 233 could be used because the absence is due to the “usurpation of the presidential office, which has left the position empty.”

Maduro's presidency is considered invalid because the elections were conducted in an illegal manner and were not recognized as valid. This makes it so the presidency is constitutionally empty, allowing for the National Assembly to take over the role of president. The National Assembly was democratically elected.

Quoting this aticle:

The electoral conduct has been described as being fraudulent, with the call for an election by the pro-government Constituent National Assembly being declared unconstitutional in the first place, especially when the body moved the election date ahead from December to April. The National Electoral Council (CNE), which is charged with overseeing elections in Venezuela, is also controlled by Maduro sympathizers. The Bolivarian government has also been accused of excluding opposition candidates, handpicking candidates, voter intimidation, vote buying, and offering food to those who vote for President Maduro. No recognized electoral observers were said to be present for the elections.

If you doubt any, and I mean, really, any point from that paragraph, just ask or google it. It's all VERY well documented. Guaidó's position as the interim President of Venezuela is not unconstitutional or undemocratic. Maduro's presidency is.