1/ Is the procedure to edit such invoice is editing it as PDF from the "customer invoice" vue ?

You can, but you won't get anything looking like an invoice.
It is just the export of detail view of invoice.

2/ When using this link i got an error : "2020-02-13 10:32:36.854 ** ERROR ** [V8.3.2] cause = Undefined property: Bill::$tax" and the pdf is not displayed properly

It seems you have an old "Bil" template, that was removed because everyone was requested to have it adapted to his onw way to do.
So it is not maintained any more.
You can find the code in report/object/Bill.php.
Either you can fix the code to have it work and fit your need, of delete this file to retreive defaut export.

3/ Is there a native way to customize the invoice with template (Company Logo, texte befre/after the billing details, ...) or should we use the pluggin Template report ?

This can be done thanks to our "Template rpoert" plugin (that is not really template for reports, but template for exports.

4/ I woud like to use the "Invoice according to produce workload" mode, on project with tens of people working on tens of activities ... The only way i found to generate a bill is to add for each people, as many line as existing activities for a dedicated period, it tooks me hours. Is there a way to automatically add non-empty billing lines (including all people and all activities) just selecting a period ?

No, there is no tother way, but you can simplify it as you have to select an "Activity price" that only depneds on project and activity type, so not one per activity and per resource.

Thank you for your answer,

For 1 and 3/ Actually, customizing the object/Bill.php is really acceptable and the result is not so far that what i expected.

For 2/ I'm however still unable to retrieve the tax field from Bill Object and i do not undestand why:

$bill = new Bill();
$bill->tax; returns an error "Undefined property: Bill::$tax"" while the tax column exists and value is not null in the bill table.

4/ for one dedicated project, I have several activities with different prices.and each ressource can work on all activities. Then I think i can not simplify (except by using the catalog, but in this case, I have to manually set the quatity for each activity)

For 2/ I'm however still unable to retrieve the tax field from Bill Object and i do not undestand why:

$bill = new Bill();
$bill->tax; returns an error "Undefined property: Bill::$tax"" while the tax column exists and value is not null in the bill table.

Don't focus on db column name.
We use an object mapping
you have
$bill->taxAmount;
$bill->taxPct;