Smartphone

Yes.  But mediocre cell phone cameras make it harder to shoot high quality images.  Good Stock Photos have to be technically perfect, not just so, so for social media.  See links below.

• https://toughnickel.com/self-employment/Create-Passive-Income-With-Stock-Photography-On-Your-Mobile-Phone
• https://helpx.adobe.com/stock/contributor/help/quality-and-technical-issues.html
• https://helpx.adobe.com/stock/contributor/user-guide.html/stock/contributor/help/photography-illustrations.ug.html
• https://helpx.adobe.com/stock/how-to/tips-stock-image-acceptance.html?set=stock--fundamentals--adobe-stock-contributor

Good Entry Level DSLRs:

• https://www.techradar.com/news/best-entry-level-dslr-camera
• https://photographylife.com/best-entry-level-dslrs

Hope that helps.

Yes, I have had images from my iPhone8 accepted. I do put them through the same Lightroom/PS editing and scrutiny as the images from my Canon DSLR's and never post something straight out of the camera.

Yes, sure. You need just to know that cell phone pictures will be vetted as stringent as pictures from the best camera of the world. But there are cell phone pictures in the database that sell.

If you are new to stock, you should consider these resources: https://helpx.adobe.com/stock/contributor/tutorials.html
Please read the contributor user manual for more information on Adobe stock contributions: https://helpx.adobe.com/stock/contributor/user-guide.html
See here for rejection reasons: https://helpx.adobe.com/stock/contributor/help/reasons-for-content-rejection.html
and especially quality and technical issues: https://helpx.adobe.com/stock/contributor/help/quality-and-technical-issues.html