What has Bernie done in his time in leadership that illustrates what he'll do as president?
I know what he espouses. What's he done and got through the bureaucratic machine for which I'm a layman?
Part of Bernie's appeal is that even though he has been in the senate for 30 years he hasn't been part of the establishment of either party. Which makes it harder to pass legislation, no doubt, although he has done admirable work on admendments to bills and has at least 1 veterans care bill passed. It's also been noted by republican legislators that Bernie has regularly offered to leave his name from bills to help them get passed.
This will likely mean that he will have a harder time getting everything he wants passed, but his longevity in the senate as well as mixing experienced people around him and the grassroots policy advocate groups that make up his campaign he will be able to get some stuff passed. It should also be noted that much like the republican party saw the way the winds were turning in regards to Trump there will be a similar effect within the dems after this primary campaign.
As for what he will try to do as president then you only have to look to what he has said consistently over his 30 year career. Which is help working class people, fight for equality, and tackle the climate emergency. People may not like Bernie's agenda but even his fiercest critics believe he's sincere about it.