For Conventional, there are two basic ways that income is evaluated - properties being placed into service (such as a new property being purchased), and properties already in service. For properties being placed into service for the first time, 75% for the gross rent (based on the appraisal/1007) is used. 25% is assumed as expenses/vacancy.
For properties already in service, the bottom line on Sched E (or however you're reporting) is used. Start with the bottom line, then add back mortgage interest, property taxes/ insurance/HOA fees (assuming they are escrowed), and any noncash and onetime/extraordinary expenses, such as depreciation. Once you have this figure, net it against the mortgage payment for that property (if there is one). If the net figure is positive, meaning there is positive cashflow, it's added to your gross monthly income. If it's negative, it's considered to be a liability/debt and feeds into your DTI. If there is no mortgage payment at present, escrow items are not added back in the above explanation.
It's more nuanced than this and there are many exceptions/caveats, but this gives a high-level explanation.
For DSCR loans, in most cases, the raw rental income is used, whether based on the rental appraisal (1007) or an actual lease. Some lenders compare this to only the actual mortgage payment and some include an expense factor. DTI and personal income are not considered.