If I delete photos in iCloud, will they remain in the photos app.

It will depend on your settings for iCloud.

When you delete a photo in iCloud, directly in the Photos.app on your iCloud webpage www.icloud.com , the phto will be deleted from all your devices, that are syncing with iCloud Photos, using the same AppleID for iCloud. This is to be expected, as the main purpose of iCloud Photos iss to keep the Photos Libraries on all devices in sync and identical with iCloud Photos.

If you want to see different photos on your devices in Photos than in iCloud Photos, do not enable iCloud Photos on the devices. Use iCloud Photos only, if you want the syncing feature, an identical Photos Library across all your devices, with your photos stored off-site in iCloud, and mirrored on all your devices.