Google Photos will back up unlimited quantity photos and videos with a cap on quality:
Back up unlimited photos and videos for free, up to 16MP and 1080p HD.
Access them from any phone, tablet, or computer on photos.google.com –
your photos will be safe, secure, and always with you.
Amazon Photos: If you have Amazon Prime, Amazon ...
Yes, OBS Studio is your best option here, as it's very customizable.
OBS Studio here also.
Geogebra is a fantastic free option for the majority of mathmatical things. However, making graphs simply via drawing may prove difficult, and I couldn't find any software that would support this, so you'll probably have to use a simple painting software such as PAINT....