Twenty is an open source CRM with about 55k stars on Github.
Their website mentions the features and how good it is.

I am also interested to know what it can't do and its shortcomings.
If you tried it and didn't like it, why and what turned you off from using it.

I have seen people use apps like Word, OneNote, Excel and other note taking apps for taking notes outside of the CRM. What's your use case and why aren't you doing it inside the CRM? Is the CRM software lacking a notes section?

If you're making a call to a regular phone number in the US, how is the call getting recorded, transcribed and summarized back into the CRM? Is a VOIP system or a dialer being used?
Is Whatsapp being used?

I am asking about the recording part in particular when making a regular phone call. I know once a recording has been captured, voice to text and AI can do the transcribing and summarization. Does the CRM have like an api to receive the text including the metadata, so it knows where to place it?

Most of the CRM software is web-based. There are some free open source ones like EspoCRM. Some companies shy away from them because they require IT skills, monitoring, maintenance, backups, which are hassle or expensive for small companies.

If you're a solo CRM user or a small company with a few users, would you use a free desktop CRM software if it were as easy to install as any desktop app? If you're the only user, the app and the database will be on your computer. If you were a company, each user will have it installed on their work computer. The database can be hosted anywhere. A computer in the company with no hosting fees, or hosted in the cloud (Azure or AWS) or hosted at a hosting company with as little as $5 for shared hosting with backups. This option will be Windows only, so no Mac or Linux.

There will also be a web based version with the same features.

But I am inquiring right now about if there's interest in the desktop version if it means it's hassle-free to install, host and use. It's free as well.

The business model is making money from advanced customizations.