https://www.newadvent.org/summa/3078.htm

Here's my attempt to summarize his beliefs (please correct it if it's wrong):

It is a sin for people to lend at interest - things like interest on credit card purchases, car loans, student loan, and mortgages, are considered to be sinful for the lender who lends and expects interest back on the loan, because they are not producing anything and so to take such interest is a kind of theft.

The thief who takes such gains at interest is bound to pay back what was stolen (but is in justice entitled to the principal or the thing or amount loaned if they lawfully possessed it).

The borrower does not sin if they take a loan at interest of necessity: a poor student who must take a student loan to get an education may do so without sin (although it might be worth asking the question if it is possible to fund education without having to take on such loans at interest).

It is lawful to be a landlord and make money from renting things since one owns those things and provides a tangible good or benefit.

It is lawful to make money from investments, since one is a part owner of such a company and such companies produce value.

Not covered in the text: it may be a sin to invest in companies who openly promote sin like "woke" companies, because of one's cooperation with the sin as a part owner of the company (saw another article mention "material" versus "proximate" cooperation in others' sins, does anyone have any input on this?)

Covered but mentioning it for emphasis: The Christian ideal seems to be the opposite of usury, which is to lend without expecting interest nor the principal (what was loaned) to be paid back: "But love ye your enemies: do good, and lend, hoping for nothing thereby: and your reward shall be great, and you shall be the sons of the Highest; for he is kind to the unthankful, and to the evil." (Luke 6:35) It would be lawful however to ask for what was loaned back though - one is not bound to lend without expecting anything back, but this is an ideal to strive towards beyond what is required in strict justice - this is "going the extra mile" morally.