Politicians don’t receive money directly from lobbyists and donors. Doing so would be a violation of rules against taking bribes.

Lobbyists and donors give money to campaigns, which require untold sums of money these days, and a congresspersons power within the party is directly correlated to their ability to fundraise for the party.

As to their actual personal means of a congresspersons wealth, they’re paid fairly high salaries (close to 200k per year, which, despite the expectations of this forum is quite a bit for the US) and yes, they have the ability to engage in their own investments and business dealings on the side. Not to mention their incredibly generous pensions.